The movie soundtrack typically refers to the music recorded for a film. It can be a score (orchestral music played over the action), songs performed as part of the film (such as in a musical) or songs heard by characters in the background (such as on the radio in their cars or in the restaurant where they eat).
Overall Themes
The soundtrack can emphasize the overall theme or atmosphere of a film. John Williams's "Jaws" soundtrack, for instance, stresses a wide-ranging theme of danger and dread.
Specific Themes
Music used during a particular scene can stress certain emotional cues, such as when the hero is in danger or when two lovers join in an embrace.
Character and Personality
A movie soundtrack can cue specific traits of a character's personality. "The Imperial March" from "The Empire Strikes Back," for example, stresses the power and menace of Darth Vader.
Bridging
Soundtrack music can connect multiple shots or scenes together, providing a common thematic link.
Plot
In some instances, a song on the soundtrack can serve a particular purpose in the plot. This often occurs in musicals, and was also used in the movie "Close Encounters of the Third Kind," in which the movie's theme actually depicted musical notes sent from an alien ship.