When inner clarity arrives, all of one's sorrows come to an end. For the person whose mind is serene, the intellect soon becomes firmly settled.
Explanation
Krishna is continuing his description of the person of steady wisdom. Having said that self-mastery brings inner clarity (prasāda), he now says this clarity ends all sorrow (duḥkha) and quickly steadies the intellect (buddhi), the faculty of discernment. Traditions differ on whether this clarity is mainly the fruit of disciplined action and devotion, or of knowledge of the Self, and the verse itself does not decide between them.