Flint is an item used to craft basic tools. It can be found on the ground or by mining smooth or gold veined Boulders with the Pickaxe. Flint is not flammable and easy to find on the surface, although eventually it can be used up. More can be found by venturing into a Cave, however.
Flint is one of the first and most needed items in the game and should be sought after as soon as a new game starts. It is recommended to always have a decent number of flint at all times in the case the player needs to make basic Tools such as an Axe or a Pickaxe. Combat wise, the Spear can be built.
Flint is a renewable resource. In Reign of Giants, it can be farmed from Moleworms, Rock Lobsters, and from Earthquakes in the caves. In Shipwrecked, it's rather more difficult, being generated by the Slot Machine and from mined or hatched Dragoon Eggs. Don't Starve Together also has renewable Flint via Petrified Trees.