A stave is the crosspiece between the legs of a chair or a wooden slat forming the side of a barrel. In music, it's the five lines you write notes on.
Stave comes from the word staff, as in a walking stick. You're most likely to encounter the word stave if you're learning woodworking. If someone "staves in" a door, they've broken a hole in it. You also might hear the idiom "stave off," which means to hold off for a short time.