तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः।
ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा।।17.23।।
17.23 Om-tat-sat [Om, That, Existence: Om iti brahma, Om is Brahman (Tai. 1.8.1); Tattvamasi, Thou art That (Ch. 6.8.7); and Sadeva somya idamagra asit, This was Existence alone in the beginning, O amiable one (Ch. 6.2.1)-in these texts Brahman is indicated by the words Om, tat, sat.]-this is considered to be the threefold designation of Brahman. The Brahmanas and Vedas and the sacrifices were ordanined by that in the days of yore.
17.23 Here Brahman means the Veda. It is the secondary meaning of the expression, especially of the ritualistic portion of the Veda. The three-fold expression Om Tat Sat is connected with the Brahman or the Veda. Sacrifices and similar rituals are prescribed in the Vedas. These expressions Om Tat Sat are used in these Vedic rites. The connection of Om is that it should be invariably used at the commencement of the recitation of Vedic hymns. The syllable Tat and Sat indicate that these rituals are worthy of honour. The Brahmanas are those who are to preserve Vedic study as also the Vedas and the sacrificial rites ordained in them. All these were created by Me in the past. Sri Krsna elaborates in the next verses the nature of the connection of these syllables with the Vedic rituals, beginning first with Om.
Om tatsaditi nirdesho brahmanas trividhah smritah; Braahmanaastena vedaashcha yajnaashcha vihitaah puraa.
om tat sat—syllables representing aspects of transcendence; iti—thus; nirdeśhaḥ—symbolic representatives; brahmaṇaḥ—the Supreme Absolute Truth; tri-vidhaḥ—of three kinds; smṛitaḥ—have been declared; brāhmaṇāḥ—the priests; tena—from them; vedāḥ—scriptures; cha—and; yajñāḥ—sacrifice; cha—and; vihitāḥ—came about; purā—from the beginning of creation