Things that are under something else are beneath it. A star gazer is beneath the night sky, and tropical fish swim beneath the surface of the water.
When one apple hangs lower on a tree than another, the first apple is beneath the second. You can also use the adjective beneath in a figurative way, to describe something that ranks lower or has a low worth or status: "He didn't want to play soccer at all, because he thought the Junior Varsity team was beneath him." The Old English root is beneoðan, combining be-, "by," and neoðan, "below."