Imagine you have some pieces of paper and a matchstick. When you put the matchstick to the paper, it catches on fire and burns, right? That's because paper is combustible, which means it can easily catch fire and burn.
Things that are combustible, like paper, wood, or gasoline, can catch fire easily when they come in contact with something hot, like a flame or a spark. When they burn, they release heat and light. It's important to be careful around things that are combustible so that we don't accidentally start a fire.