तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः। ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥

oṁ tat sad iti nirdeśho brahmaṇas tri-vidhaḥ smṛitaḥ brāhmaṇās tena vedāśh cha yajñāśh cha vihitāḥ purā

" Aum Tat Sat," this has been considered to be the threefold designation of the Eternal. By that were ordained of old Brâhmanas, Vedas and sacrifices.

In plain words: Krishna says the three words "Om Tat Sat" are a sacred name for God, the Eternal. Long ago, priests, holy books and acts of worship all came from this.