The small pieces of wood you use to start a fire are called kindling. When you go camping, you can gather kindling, little twigs and branches, to use in a campfire.
Kindling helps a fire get going because it's small and dry, easily flammable. Once the kindling is burning, it can ignite the larger pieces of wood in your fire. You can also use this word to mean "the act of lighting," so you could say, "Kindling this fire is so hard after all that rain we had." Kindling comes from the Old Norse root kynda, "to light a fire."