Commentators universally agree that this verse uses the metaphor of a fire. The Holy Spirit's work in a believer's heart is like a flame that provides spiritual light, warmth, and passion. This fire can be dampened or "quenched" through personal neglect of prayer, indulging in sin, worldliness, or allowing a spirit of gloom to take over. The command is a call to actively cultivate and fuel the Spirit's work in your life.