The background is the physical space. That sits behind the main action that is taking place in your middle ground area. And should not overpower or take away from the interest in the main, middle area.

A background actor, or extra, is a nonspeaking role; extras appear frequently in the background of television or film productions, intended to round out the scene for realism’s sake. They could be spectators at a baseball game, audience members at a comedy club, or just people strolling through Central Park—these are all common background roles.