This is during the Main Street level. At the start is a flame to catch yourself on fire, which doesn't hurt you but is only temporary. Your goal is to use this to light a series of blue gas leaks along the street until you come to the bosses. When you do, just light yourself on fire again and get close to them to blow them up. If you miss lighting a leak, just go back to the last one you lit and try again, no need to start over.