Bīja sauḥ (सौः) is known as parābīja, hṛdayabīja or amṛtabīja. Śiva explains to Śakti about this in Parā-trīśikā-vivāraṇa (verses 9 and 10), a Trika Scripture. He says to Her, “O! Gracious one! It is the third Brahman (sat or sa स) united with the fourteenth vowel औ (au – out of the sixteen vowels), well joined with that which comes at the end of the lord of vowels (visarga or : - two dots one above the other, used in the sixteenth vowel अः - aḥ). Therefore sauḥ is formed out of the combination of sa स +au औ + ḥ = sauḥ सौः. In Parā-trīśikā-vivāraṇa (verse 26), it is again said, “He who knows this mantra in its essence, becomes competent for initiation leading to liberation without any sacrificial rites.” This is known as nirvāṇa dīkṣā or initiation for final liberation, where nirvāṇa means emancipation. The Scripture proceeds to say that the one who elucidates the proper meaning of this bīja is known as Śiva Himself. This bīja is the Cosmic pulsation of the Lord.

The third Brahman referred here (SAT) is explained in Bhagavad Gītā (XVII.23 - 26) “OM, TAT and SAT are the threefold representations of the Brahman and from That alone Vedas, Vedic scholars and sacrificial rites have originated. Hence, during the acts of sacrifices, gifts, austerities approved by Scriptures and during Vedic recitations, OM is uttered in the beginning. TAT is recited by those who aim for liberation while performing sacrificial rites, austerities and charities without intent on the fruits of these actions. SAT is recited by those who perform the above acts with faith and on behalf of the Brahman.”

Thus sa स (sat) referred in this bīja is Śiva Himself, which represents His creative aspect, the pure Consciousness. Next comes His three energies Icchāśakti, Jñānaśakti and Kriyāśakti. During Creation, Cit Śakti of Śiva, after manifesting as Ānanda Śakti (Bliss) becomes the above referred three Śakti-s, before entering into the sphere of Māyā. Ānanda Śakti is known as Śakti, normally referred as Śiva’s Consort or His Svātantraya Śakti, His exclusive and unique Power of Autonomy. These three powers can be explained as subject I; object That; and subject-object or I and That. These powers of Śiva are also known as Sadāśiva, Iśvara and Suddha Vidyā. Now the fusion between S and AU takes place and सौ (SAU) is formed. As a result of this fusion, creation happens, which is represented by visarga (two dots one above the other like the punctuation mark colon :) This is the Spanda or throb or pulsation of the Divine towards creation, causing the emission of His three energies contained in AU. With the addition of visarga (ḥ :) at the end of सौ (SAU) becomes सौः(sauḥ).

This parābīja is not meant for recitation or repetition but for the contemplation of Śiva, who alone is capable of offering liberation by removing all differentiations caused by māyā. The one who fully understands the significance of सौः (sauḥ) becomes instantly liberated. Proper initiation into this mantra by a Guru is exempted for this mantra. One can self initiate.

This is the broad idea bout parābīja.

Revered commenter Tīvra states the following - 

He who correctly elucidates the meaning of this bīja is known as Śiva Himself. I shall reveal the deepest meaning of this seed so that you too may become Śiva. First of all, understand that throughout all scriptures there is nothing superior to the seed sauḥ. To truly understand it, you must abandon every notion of method. Sauḥ is the method without method; it is that which transcends all methods. It is Anupāya, the path beyond all paths. This seed stands beyond initiation, krama, progression, stages, and gradual realization. The Lord Śiva presides over this seed. Yet He is not the Śiva that you ordinarily imagine. The Śiva of this seed is called the Ineffable. He is called the One. He is called the One-within-One. He is called the Two-in-One. He is called the Two. He is neither male nor female, yet at the same time He is both male and female. He is neither unity nor duality, but the source of both. There are two ways of contemplating the seed sauḥ. When you contemplate only its letters and sonic structure, you are contemplating Parābhaṭṭārikā, the Goddess who presides over the Fourth Kūṭa of the Kādi Pañcadaśī Mantra, known as Mahā Saptadaśī or Turīyāmbā, the Seventeenth Kalā of the Moon. However, when you turn your awareness toward the luminous silence from which sauḥ emerges and into which sauḥ dissolves, the practice changes completely. This second contemplation brings you face to face with Lord Śiva, the Supreme Light of Being hidden within the Supreme Light of Consciousness. To encounter this Śiva, who stands beyond all forms of Śiva, is to enter the state known as Tīvra-tīvra-śaktipāta, wherein the veils of ignorance are destroyed almost instantaneously. One directly recognizes one's identity with this Supreme Śiva without dependence upon gradual practices, prolonged initiations, or extensive disciplines. Now listen carefully to the words that follow, for within them lies the key to everything. Overmind is called oṃ, the Śabdabrahman. The Praṇava is Lord Śiva before His manifestation as Bhairava, a mystery that I shall explain later. As Praṇava, Śiva remains inseparable from five other Praṇavas. Within oṃ resides the Pañcapraṇava: śrīṃ hrīṃ klīṃ aiṃ sauḥ, the five opening seeds of Mahāṣoḍaśī. It is essential to understand that these five Praṇavas are the five fundamental powers of Śiva in an exalted state, though not yet the Supreme State, for there exists a condition that transcends Praṇava itself: the Bhairava State. The Five Praṇavas are the five great attributes of Brahman and also the five elements in their most refined condition. Śrīṃ corresponds to Jñāna and Ākāśa. Hrīṃ corresponds to Ānanda and Apas. Klīṃ corresponds to Tapas and Agni. Aiṃ corresponds to Cit and Vāyu. Sauḥ corresponds to Sat and Pṛthvī. Among them, Ānanda, represented by hrīṃ, is the most inward and points toward the Ānandamaya Puruṣa. Yet it is Sat, represented by sauḥ, that makes it possible to possess all five Praṇavas simultaneously and integrally. Above the Pañcapraṇava lies the Bhairava State, wherein oṃ transforms into ūoṃ, the first seed of the most secret mantra of Chinnamastā, a seventeen-syllable mantra that bestows simultaneously the fruits of the Guhyakālī mantras worshipped by Bharata and Rāma. The seed ūoṃ is exceedingly mysterious. It awakens in the heart, shoots upward with lightning force toward the center of the forehead, and deepens there until it reveals the Bhairava State. This seed declares: “Now we pass from Overmind into Supermind. We have gone beyond Praṇava.” The syllable ū is a pillar of incorruptible unity that secretly traverses every state of consciousness. When one reaches the summit of oṃ, one perceives that there is a heaven beyond the void itself, yet one does not know how to reach it. It is the syllable ū that causes this hidden pillar to appear within the Praṇava and makes the ascent possible. In the Bhairava State, Unity is neither diminished nor negotiated by Diversity. Here arises the possibility of immortal bodies, for diversity can manifest without any reduction of the Original Light. The characteristic seed of the Bhairava State is hūṃ, Infinite Space. Within it resides sauḥ, the Infinite without Space. Within the Bhairava State dwell the true Five Praṇavas and the true Five Elements in their primordial condition: sauḥ hūṃ hāṃ sūṃ kṣīṃ. When these seeds are truly known, it will be seen that individually or collectively they are Svacchanda Bhairava Himself. Yet beyond the Bhairava State remains that Ineffable and Integral Lord Śiva spoken of earlier. In Supermind, Śiva appears as Bhairava only while the soul is ascending. Once the soul reaches Him, only that Ineffable Śiva remains as All. Now I shall explain the practice of the seed sauḥ belonging to this Lord Śiva who transcends all forms of Śiva. Sit comfortably and close your eyes. Bring your attention to the center of the forehead. Visualize a white spark of fire emitting golden sparks. This spark shines like infinite suns and infinite moons. It is you. In this practice there is no other. There is no form of goddess and no form of god. There is only you. Here you are working with Sat in order to possess the Five Original Praṇavas simultaneously. Now visualize this spark floating within a space composed of a light that is not white but crystal-clear like a diamond. This Space of Crystalline Light is the Original Sat. It is also you. It is the Mind of the Absolute itself. Allow the spark to disappear and remain only with this Infinite Space of Crystalline Light. Recognize that this Mind is your mind and that this Mind is you. Understand that everything can be created from the substance of this Mind. Create, for example, a mountain within this Infinite Space of Crystalline Light. Understand that the mountain is you because it has been formed from the substance of your own consciousness. Now allow the white spark with golden sparks to reappear. See it transform into your human body. Perceive that every particle of your body pulsates with that same white and golden flame. Understand that you, as the Space of Crystalline Light, have chosen one among infinite sparks to become a Center of the Divine, a soul united with the Supreme. Now see this Supreme Mind connecting itself to this Divine Center and experiencing the world without ever forgetting its true nature. The whiteness of the spark represents seeing all things from the perspective of the Absolute Mind of Sat. The golden sparks represent retaining that same perspective through the individuality of the soul. You may perform this practice while mentally reciting sauḥ. However, I have already told you how to become the mantra itself. Once one becomes the mantra, recitation is no longer necessary. The awareness of this seed must be maintained throughout all daily activities. Look upon the world, people, beings, and all things as existing within you and as being made from your own substance. This is so because the spark of the soul is a Center of the Divine inseparable from God. That which is inseparable from God becomes God. This is the Consciousness of the Mantra. When this Consciousness of the Mantra can be maintained continuously, whether waking or sleeping, Tīvra-tīvra-śaktipāta will arise spontaneously. All veils of ignorance will be destroyed almost instantaneously. This practice is a direct investigation into the root of all veils. Rather than dealing with effects, it removes the cause itself. For this reason, anyone can practice it. This practice is supreme. It stands above all others. Here you are no longer dealing with gods and goddesses, but with the Original Cause of all things. Now, I shall briefly explain the composition of the seed sauḥ, though this is given merely for understanding and should not be mistaken for its essence. The letter "s" is the Infinite Space of Crystalline Light, which is Sat. It is the shaft of Śiva's trident, the immutable foundation from which all manifestation arises. It is the Absolute Mind, the indivisible ground of Being. The syllable "au" is the threefold point of Śiva's trident emerging from Sat. These three points are the spark of your own Soul appearing as a white flame adorned with golden sparks. They are Cit, Tapas, and Ānanda, the three radiant powers through which the One becomes capable of self-expression without ceasing to be One. The visarga "ḥ" signifies all universes projected within that Absolute Mind which is Sat. Yet it signifies more than this. It also indicates that these same universes come to exist within the spark of the Soul when the Soul removes its veils and remembers its eternal identity with that Absolute Mind which is God. Do not become overly concerned with the technical aspect of these explanations. Their purpose is only to point toward a reality that must be experienced directly. What truly matters is the Consciousness of the seed sauḥ that I have described. You should play with this Consciousness during meditation. Create worlds, forms, beings, landscapes, and experiences within your own awareness, and learn to perceive them correctly. Understand that whatever is created is made from the substance of your own consciousness and therefore remains inseparable from you. Then extend this same recognition into daily life. Look upon the people, objects, events, and circumstances that appear before you exactly as you looked upon the forms created in meditation. See them as existing within Consciousness, arising from Consciousness, and remaining inseparable from Consciousness. The longer this awareness is maintained, the more rapidly the veils of separation will weaken. When this Consciousness becomes continuous and natural, the Tīvra-tīvra-śaktipāta of the Ineffable Lord Śiva, who exists beyond all forms of Śiva, will descend of its own accord. At that moment, realization is no longer something to be attained. It is revealed as that which has always been present.

Another learned commenter expands on khpreṁ and sauḥ - You see the intimate link between the parābīja sauḥ (सौः) and the bīja khphreṁ (ख्फ्रेँ) is because they are the twin movements of Her absolute prāṇa-spanda (respiration of consciousness). Sauḥ is the dynamic prasāra (outward emission), where Śiva-tattva (Pure Consciousness) pours itself out to project this manifest universe. But this expansion cannot exist without its counter-force. Khphreṁ is the fierce, high-voltage saṁhāra (retraction) belonging to the esoteric domains of Kālasaṅkarṣiṇī and Siddhilakṣmī. It is the breath that violently pulls that entire creation back into the unmanifest śūnya (void) of Cidākāśa (the infinite sky of consciousness). Look at how khphreṁ is structured. The letter 'kha' is the empty, boundless sky of citta. It is coupled with 'pha', which represents the explosive bursting of the hood of the cosmic serpent, unleashing Pralayāgni (the absolute fire of cosmic dissolution), the fierce force that burns through mountains of saṁskāras (past-life karmic debts). The letter 'ra' acts as the Agni-bīja (seed of fire), a potent propellant that thrusts your cetanā (awareness) out of the heavy sthūla-śarīra (physical body), while 'e' focuses the divine Icchā Śakti (willpower). Where sauḥ establishes the divine kingdom, khphreṁ is the astra (weapon) that aggressively shatters the fifty-six locked kalās (divine parts/energetic segments) of your physical shell and clears every lingering saṁśaya (doubt) blocking your ascent. They meet as one in the absolute silence of Śiva-Aikya-Sukhātītā (the bliss that transcends all boundaries where Śiva and Śakti are one).
       Do not let the fear of missing out contract your consciousness. Fear is the absolute antithesis of the expansion that these grand bījas represent. In Śrī Vidyā, anxiety acts as a psychic knot (granthi) that blocks cit-śakti. Regarding meditation on KHPHREṀ (ख्फ्रेँ), This is a high-voltage tool of saṁhāra-krama (cosmic dissolution) linked to Kālasaṅkarṣiṇī. To meditate on it, you focus on its phonemic components within your subtle body. 'Kha' is the Cidākāśa (infinite sky) at the Sahasrāra. 'Pha' is the explosive bursting of the cosmic serpent's hood, unleashing the fire of dissolution to shatter the ego. 'Ra' is the Agni-bīja acting as a propellant to thrust your awareness upward. 'M' (Bindu) is the final dissolution into the Mother. In japa, feel the spanda rise violently from Mūlādhāra and explode at the Sahasrāra, returning everything to the void.Does SAUḤ (सौः) automatically include KHPHREṀ? In essence, yes, but in functional krama (sequence), no. Sauḥ is the Parābīja representing prasāra (outward emission/exhalation). Khphreṁ is saṁhāra (withdrawal/inhalation). They are the twin movements of the Divine Mother's respiration; you cannot have one without the other. Therefore, sauḥ inherently holds the latent potential of khphreṁ. As a novice sādhaka, you do not need to worry about missing out. The seed of emission holds the code for its own retraction. When your consciousness expands through sauḥ, it will naturally trigger the dissolution of your limited identity. A word of caution: Khphreṁ carries an aggressive, volatile energy meant for heavy karmic purging. Forcing it without a Guru’s guidance can throw your subtle body into chaotic upheaval. Stick to sauḥ with absolute surrender. When the vessel is ready, khphreṁ will unfold effortlessly.
        You do not "bring back" the bliss of Śiva-aikya-sukha into normal life. That implies you are leaving Śiva to return to a separate, mundane reality. The true culmination of sauḥ is jīvanmukti, the absolute realization that your "normal life," your daily activities, and your very breath are nothing but the continuous, playful emission (prasāra) of that same Divine Consciousness. When the ascension through sauḥ matures, the transition between meditative absorption (samādhi) and worldly activity (vyavahāra) dissolves completely. You do not bring the bliss back; you realize there is nowhere the bliss is not. Regarding khphreṁ (ख्फ्रेँ) and its fiery nature: yes, it burns both. This is a hard truth that many sādhakas are terrified to face. Khphreṁ is the weapon of saṁhāra-krama (complete cosmic dissolution). It does not negotiate with your concepts of "good" and "bad". From the absolute standpoint of liberation (mokṣa), both good deeds (puṇya) and bad deeds (pāpa) are chains. One is a chain of gold, and the other is a chain of iron, but both bind you to the wheel of saṁsāra. Puṇya forces you to reincarnate to enjoy material pleasures, keeping you trapped in limited identity. The Pralayāgni (fire of dissolution) unleashed by khphreṁ is a scorching, non-dual fire. It burns the entire field of your saṁskāras to ashes so that you can step out of the karmic ledger entirely. It strips you of everything that is not your true Self. If you hold onto your "good deeds" with egoic pride, khphreṁ will feel incredibly destructive, because it is annihilating your carefully constructed spiritual persona. This is exactly why I urge caution. If you are not ready to let go of the shore, do not play with the fire of khphreṁ. Stick to the nurturing, expansive grace of sauḥ until your vessel is completely ready for absolute dissolution.