All unwanted parts of an audio recording.
One or more microphones on the set will pick up not only the dialogue of the actors, but also their movements, steps and actions. This is all unwanted noise, floor creaks during a tracking shot, buzzing lamps, ballasts, humming generators, wind machines, etc. All sound events that foreign to the action and dialog in the frame could potentially remove the viewer from the illusion of the film, i.e. suspension of disbelief.