यज्ञे तपसि दाने च स्थितिः सदिति चोच्यते। कर्म चैव तदर्थीयं सदित्येवाभिधीयते ॥

yajñe tapasi dāne cha sthitiḥ sad iti chochyate karma chaiva tad-arthīyaṁ sad ity evābhidhīyate

Steadfastness in sacrifice, austerity and gift is also called " Sat ", and action for the sake of the Supreme is also named " Sat."

In plain words: Krishna says that sticking faithfully to worship, self-discipline and giving is also called "Sat," or good. Any action done for God is called "Sat" too.