A rafter is a long board that supports the peak of a roof. If your bedroom is in the attic of your house, you can lie in bed staring up at the rafters.
Rafters are a type of beam, a heavy supportive board that holds up whatever is above it in a building. Some make the distinction that rafters support a roof, while beams support heavier loads (such as the floor of an upper story in a house). Most rafters are made of wood, and sometimes they're left visible in the ceiling as a design element. The word comes from a root related the the Old Norse raptr, "log."