A cracker is a snack. It’s a thin, savory, crisp biscuit, like a saltine. It’s also anything that cracks, like a firecracker or a computer code cracker, or of course, that thin, savory snack that cracks when you break it.
Besides a baked wafer, another kind of cracker is a noisemaker — either a small firework that makes a sharp bang, or the kind that snaps loudly when you pull its colorful paper wrapper apart. Cracker can also be used as an offensive term for a white person, particularly one who's Southern and poor. This meaning comes from the sense of crack as "brag," and a cracker, therefore, as a "bragger."