The word "sat" is used for what truly exists and for what is good. And likewise, O son of Pritha, the word "sat" is applied to an action that is praiseworthy.
Explanation
Krishna is explaining the three-part sacred phrase "om tat sat", and here he turns to the word "sat". It stands for reality or existence (sadbhāva) and for goodness (sādhubhāva), and it is also applied to praiseworthy action (praśasta karma). The verse gives no single definition, so readers take "sat" as covering what is real, what is good, and what is rightly done.