ॐतत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः ।ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा॥
oṃ tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥbrāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā
Word by word
The verse says
Om Tat Sat - these three words have always stood for the absolute. From them, long ago, came the priests, the scriptures and the offerings.
In plain words
Three syllables, and the chapter changes gear.
After all that sorting, Krishna gives a kind of seal. Oṁ tat sat - roughly: yes, that, real.
They are not an instruction. They are a way of pointing an act at what it is for, and the next three verses take them one at a time.
Where you've felt this · in study
A child learns the first line of a prayer years before she learns what any of it means. She says it at the same hour every evening, standing in the same place. At twenty-six, in a difficult week in another country, the line comes back whole. She still could not translate it. It still did the thing it was for.
Going deeper
Three syllables standing in for the absolute looks like mysticism and is closer to engineering.
Oṁ is the sound of assent - the yes that comes before anything in particular has been said. Tat means that, pointing away from whatever is in front of you. Sat means what is real, what actually is.
Put them in a row and you have the whole movement of this book in three words. Say yes. Point past the thing. Stay with what is real.
And the reason they get attached to ordinary acts in the verses that follow is practical rather than magical. An act needs an address on it. Without one it goes wherever the strongest appetite in the room is pointing, which is generally back at yourself.