Flint is an item that is primarily used for starting fires. When used on a campfire, it creates fire without fail, making it a more convenient method of firestarting than using sticks and bark. To be able to create fire, though, there must be at least two pieces of flint in your inventory. In Adventure Mode, the first island always contains two pieces of flint as open items near the rocky areas. In random islands, it's very likely that you'll find more than two pieces. If not, you may want to give yourself flint via the debug menu or create a new map. Despite it's historical use in primitive weaponary, it is not used in any crafting combination and is not related to weapons. Even though it is a mineral, it cannot be dug or mined.