A beam is a long board that's used to hold up a roof or doorway. The beams in your kitchen ceiling help support the upstairs floor just above it.
In construction, beams are made of wood or metal and are used to frame a building and provide structure. In gymnastics, a beam is used for balancing and doing tricks on. If someone comments that you're getting "broad in the beam," they're disparaging the width of your body, especially your hips. In Old English, a beam first meant a tree, and later a rafter or a ship's timber.