Tinder is one of the things you need to start a fire — it's the extremely dry stuff, like wood or paper, that catches easily and helps get your fire going. To build a campfire, you need both logs and tinder.
You might also call tinder kindling. This material is so dry and flammable that it quickly catches on fire and burns long enough to ignite the larger pieces of wood in your fireplace or outdoor fire pit. The word stems from the Old English tynder, from a Proto-Germanic root meaning "ignite" or "kindle."