This is a term with a broad range of meanings, depending upon the context. In production, it has the same connotation as 'atmosphere', meaning extras who are staged to supply detail in the form of normal human traffic in a scene. In sound, it can mean the same as 'ambience' or it may refer to relative volume.
BACKGROUND
Meaning of BACKGROUND in English
Film and video English vocabulary. Английский словарь фильмов и видео. 2012