In plain words
‘Oṃ Tat Sat’ is remembered as the threefold designation of Brahman. By that were the brahmins, Vedas, and yajñas brought forth in ancient times.
Word for word
'Om tat sat': thus the designation of Brahman is held [to be] threefold; by that the brāhmaṇas and the Vedas and the yajñas were made of old.
ॐ तत् सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतःब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यज्ञदानतपःप्रभृतीनां साद्गुण्यकरणाय अयम् उपदेशः उच्यते —
ओं तत् सत् इति एवं निर्देशः, निर्दिश्यते अनेनेति निर्देशः, त्रिविधो नामनिर्देशः ब्रह्मणः स्मृतः चिन्तितः वेदान्तेषु ब्रह्मविद्भिः । ब्राह्मणाः तेन निर्देशेन त्रिविधेन वेदाश्च यज्ञाश्च विहिताः निर्मिताः पुरा पूर्वम् इति निर्देशस्तुत्यर्थम् उच्यते ॥ २३ ॥
In plain words
So that sacrifice, giving, austerity and everything of that kind are done soundly, this teaching is now spoken —
"Oṃ tat sat" — that is the designation, exactly as it stands. A designation is whatever you point something out by; that is all the word means. And this three-part naming belongs to Brahman: the knowers of Brahman have held it in mind and turned it over in the Upaniṣads. "By that designation, in its three parts, the brāhmaṇas, the Vedas, and the sacrifices were ordained" — ordained, that is, made — "of old", meaning in earlier time. And this is said in order to praise the designation itself.
Close to the Sanskrit
For the producing of soundness in sacrifice, giving, austerity and the rest, this instruction is spoken —
"Oṃ tat sat" — thus, in this way, is the designation. A designation is that by which something is pointed out; hence 'nirdeśa'. This threefold designation by name, belonging to Brahman, has been remembered — that is, contemplated — in the Vedānta texts by the knowers of Brahman. "Brāhmaṇas, and the Vedas, and the sacrifices were ordained" — that is, fashioned — "of old", that is, in earlier time, by that threefold designation: this is said for the purpose of praising the designation.