In construction, a beam is a long piece of wood or metal that serves as the basic building block for creating a structure. Almost every building has a framework of steel or wood beams behind its walls, holding everything together.
In science fiction literature or filmography, a beam is usually a type of weapon, consisting of a long narrow ray of light capable of cutting through objects. Similar (though usually less dangerous) beams exist in real life, from laser beams to electron beams to moonbeams.
The beam of a ship is the distance across its deck in the middle, or its breadth amidships. A beam can also be the broad side of the ship, giving rise to such phrases as the "starboard beam" or "port beam."