This is one of the Gita's most beloved images. A lamp placed where no wind blows burns with a perfectly steady flame — upright, unwavering, casting an even light. That lamp is the simile for the disciplined yogi's mind.
Our minds are usually like a candle in a drafty room. Every thought is a gust — the flame leans this way, then that way, sometimes nearly going out. Meditation is the practice of finding the windless room inside. When the gusts of thought die down, the flame of awareness stands straight and still.
Notice that Krishna does not say the flame is extinguished. The mind is not shut off. It is fully alive, fully luminous — but without the flickering. Stillness does not mean darkness. It means steady, uninterrupted light.