Krishna tells Arjuna: the word "Sat" carries a wide field of meaning. It is used to express sadbhava — truth, reality, genuine existence. It is used to express sadhubhava — goodness, virtue, nobility. And it is applied to any praiseworthy action.
Consider how many words in everyday use carry "Sat" at their root. Satsang (company of the good). Satkarma (virtuous deed). Sajjana (a good person). Satya (truth). In every case, "Sat" signals that something is real, good, and worthy.
Krishna is mapping the territory of this single syllable. "Sat" covers everything that is authentic in feeling, noble in character, and commendable in action. It is the word that says: this is genuine, this counts.