It is the art of superimposing music/sound with video/media. Composing music for film is quite complex, it isn't like putting together a beat. Ok, so dr.dre scores movies right?! In theory yes, but guy's like you and me would never get away with it.
When working with film, unless stated otherwise, the music must be an accompaniment... yet still play an essential role in the video. The mighty art of transparency!
A