A pellet is a small, rounded piece of something, especially a compressed nugget of some material. Many pets eat food that comes in pellets, including some fish, rabbits, and guinea pigs.
You might feed your pet rat pellets each day, or heat your house by burning pellets made of compressed sawdust in a special kind of stove. When it hails or sleets, tiny pellets of ice fall from the sky, and your cousin's BB gun works by shooting tiny metal pellets from its barrel. Pellet is from the Old French pelote, "small ball," which has the Latin pelote, or "ball," as its root.