A bat is used in the sports of baseball and cricket to hit a small, hard ball. In baseball, a bat can be made out of wood or aluminum.
Bat can be a verb as well: when you're up at bat in a baseball game, it means it's your turn to bat with the bat. There is also an animal called a bat, a small, nocturnal bird-like mammal. People can be said to "bat their eyelashes," or flutter them flirtatiously, as well. This last meaning, from the mid-1800s, comes from an earlier sense of bat, "flutter like a hawk's wings."