As you progress through the levels, you'll encounter bugs crawling along the rope. Different bugs have different reactions to the flame. For example, the ant changes the color of the flame so you can burn different colored ropes, and the spider shoots out a Web bridge so you can access parts of the level that you could not before.
Burn the Rope is incredibly fun and addictive. You will actually feel like you're burning a rope, yet there's no need for a fire extinguisher!