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.