
In Film/TV sound, you place ambient sound effects (aka: backgrounds) in scenes to enhance the impression of the time of day, the scene's location, mood or feeling, etc. This would include effects like birds, wind, traffic, crowd voices, and many more, depending on the structure of the scene you're building. It is also the best way to describe many unspoken things in the scene subliminally.
If it's designed well, the movie fan won't give it a thought, but just accept that it was the sound that was picked up on location. There are times when background ambiences are supposed to play a more obvious role - to spook us with a scary drone, or overwhelm us with raging waters so we feel suffocated, etc - but 99% of the time, background sound effects are meant to be just that - background and invisible.

Another type of "background" in Film/TV production is the use of background performers. We don't think about them when we watch a movie, but if they weren't there, the show would seem empty. Just like background sound effects, they're vital, but we rarely notice them.
Funny that you DON'T notice something when it's there, but you DO notice it when it's not!
I'm going to be working on a feature documentary titled "Strictly Background" for about a week. It's based on ten career Hollywood extras. I found it fascinating and wanted to be a part. Hopefully Jason Connell, the creator and director of the doc will have success in getting it widely distributed. Watch the trailer.