A spandrel is a few things. In architecture, it’s the triangular space between an arch and the rectangular doorway enclosing it (see picture above). They’re almost always used as spaces for bas relief and other forms of artwork; they help give a building its beauty, provide a canvas for artists to work on. Arched bridges also have spandrels.
The term was adapted to evolutionary biology to describe a trait that evolved for one purpose (ie, for an animal to keep warm) that turned out to be useful for another purpose as well (flying). Birds evolved feathers to keep from freezing to death, not to fly; that feathers turned out to be more useful for flight had no bearing on why they showed up on birds in the first place.
So, a spandrel is a byproduct that turns out to be useful, that turns out sometimes to be more beautiful than the original space it was intended to fill. A spandrel is a placeholder. A spandrel is an unexpected evolutionary quirk. A spandrel is a testament to the simplicity, to the efficiency, of nature.