कर्मणो ह्यपि बोद्धव्यं बोद्धव्यं च विकर्मणः। अकर्मणश्च बोद्धव्यं गहना कर्मणो गतिः ॥

karmaṇo hyapi boddhavyaṁ boddhavyaṁ cha vikarmaṇaḥ akarmaṇaśh cha boddhavyaṁ gahanā karmaṇo gatiḥ

It is needful to discriminate action, to discriminate unlawful action, and to discriminate inaction; mysterious is the path of action.

In plain words: Krishna says a person must understand three things: right action, wrong action, and not acting. He says the way action works is hard to understand.