In the Bhagavad Gita, fire is metaphorically "the Fire of Desire" as well as "the Fire of Knowledge." The ravenous, insatiable fire of desire is burnt to ashes by the blazing fire of self-knowledge - a unique instance of fighting fire with fire!
Kama, desire, burns inside one's heart, always demanding more pleasure. That kama burns within due to ignorance (avidya, ajnana), in particular, failure to recognize your true nature, atma, as the source of contentment, peace (shanti), and fulfillment (ananda). That ignorance is burnt by the fire of knowledge (jnana), called jnana agni by Sri Krishna in the Bhagavad Gita.
Swami Tadatmananda is a traditionally-trained teacher of Advaita Vedanta, meditation, and Sanskrit. For more information, please see: www.arshabodha.org/