Musical Composition can refer to an original piece or work of music, either vocal or instrumental, the structure of a musical piece or to the process of creating or writing a new piece of music. People who create new compositions are called composers.

At its most simple, this could just be adding pauses or repeating the chord sequence or melody to fit the timing of a scene. In its more complicated form, it's actually changing the mood of a theme in order to meet the required emotion. The real skill in composing film music is in the arrangement.