Bhagavad Gita Chapter 17, Verse 23

ॐ तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः | ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ||

oṁ tat sad iti nirdeśo brahmaṇas trividhaḥ smṛtaḥ brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā

OM, TAT, SAT—this threefold designation of Brahman was remembered in ancient times. By that, the Brahmins, Vedas, and sacrifices were ordained.

Krishna introduces the threefold designation of the Supreme: OM TAT SAT. 'Oṁ tat sad iti nirdeśo brahmaṇaḥ trividhaḥ smṛtaḥ'—OM, TAT, SAT (iti) is the threefold (trividhaḥ) designation (nirdeśaḥ) of Brahman (brahmaṇaḥ), remembered (smṛtaḥ) in ancient times. 'Brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā'—by that (tena), Brahmins (brāhmaṇāḥ), Vedas (vedāḥ), and sacrifices (yajñāḥ) were ordained (vihitāḥ) in ancient times (purā). OM TAT SAT represents the Supreme in three aspects: OM (the sacred syllable, the beginning), TAT (That, the Supreme Reality), and SAT (Truth, Existence, the eternal). These three words sanctify all spiritual practices. Understanding this helps you recognize the importance of OM TAT SAT in making practices spiritual and meaningful.