For this yogi, whose mind is perfectly still, whose passion (rajas) has settled, who is free of stain and has become Brahman, the highest happiness comes.
Explanation
Krishna is describing the fruit of the discipline in the previous verse, where the wandering mind is repeatedly brought back to the self. When the mind is calm, restlessness (rajas, the guna of passion and agitation) subsides, and impurity (kalmaṣa) is gone, the yogi finds supreme happiness (sukham uttamam). "Become Brahman" (brahmabhūta) is read by non-dualist traditions as realizing identity with Brahman, and by devotional traditions as sharing Brahman's nature while remaining distinct.