A remark that informs an individual they have missed out on background information critical to a joke (or storey).

What your history is from.

background

The back of a stupid ugly dumb performance

If you are a background friend:
You don't fit in with any particular group of friends.
They are all closer with each other than with the background friend
People who you think about a lot, but feel don't think of you very often.

A character in a movie or play that is meant to literally just exist. These people live normal boring lives that no one cares about and thus they exist just for development and enhancement of a scene setting. These characters could include the entire animated armies that all look exactly the same and named Joe, the elderly couple at the back table chatting over a cup of coffee, or simply a random derp blurry in the background staring off into nothing while we get to see a close up of Johnny Depp's lovely face. These people are actually the true wizards because they magically appear and reappear out of nowhere, are able to switch jobs in two frames, duplicate, and can change outfits in a matter of seconds.

the random guy/girl in the background of a pic who is the center of attention