means A ray of sunlight shining through an opening or gap, especially a visible shaft of light cutting through clouds, trees, or a window.

from A plain Old English pairing: sunne (sun) plus beam, which back then meant 'tree' or 'beam of wood' before it stretched to mean a ray of light. So a sunbeam was, quite literally, a 'sun-tree' — a wooden image of light reaching down. That older sense of beam survives in the timber of your ceiling, a cousin to German Baum, 'tree.'