A lamp in a windless place does not flicker: that is the traditional image for the yogi whose mind is disciplined, practising the yoga of the Self.
Explanation
Krishna continues describing the person who is steady in yoga (spiritual discipline, union). Having said that such a person's mind rests in the Self and is free from craving, he now gives an image: a lamp in a place with no wind burns without a flicker. In the same way, the disciplined mind stays still during meditation on the Self (ātman). Traditions differ on what "the Self" means here, such as the individual self or the Supreme, and the verse itself does not settle this.