"Om Tat Sat" — this is remembered as the threefold designation of Brahman. By it, in ancient times, the brahmins, the Vedas and the sacrifices were ordained.
Explanation
Krishna has just described gifts in the mode of ignorance (tamas) and now turns to how sacred acts are made complete. "Om Tat Sat" is a threefold name for Brahman (the ultimate reality), used to consecrate actions. The verse says that brahmins, the Vedas and sacrifices (yajña) were established through it; commentators differ on whether "by it" means "from Brahman" or "by invoking this formula".