The terms “background” and “ambient” are usually interchangeable in the world of sound. This is used to describe sound that is not the focus off attention, but is just as crucial for your film. Ambient sound serves many purposes. It can be used to convey setting. For example, if you hear traffic in your background noise, it establishes a metropolitan city setting. The sound of waves establishes an ocean or a beach setting. It can also be used to determine time of day. For example, if you here crickets in the background, it establishes the time of day as the evening. No matter what, there will always be background sound in your film. Even if you are conveying part of your film where the actors are silent and standing still in an empty room, the “sound” of that empty, quiet room is very important to capture. The audience will notice if this sound is not there. We refer to this sound as “room tone”.
Also see Sound Design