voice over is the voice of an unseen narrator speaking (as in a motion picture or television commercial) b : the voice of a visible character (as in a motion picture) expressing unspoken thoughts.

A film which uses voice over, is Sunset Boulevard. This is to understand what the characters inner thoughts are without the character having to say it out loud to people for the audience to then gather an understanding of the emotions and thoughts. Sunset Boulevard also used narration so the audience can grasp the concept of what going on and gather information which some characters cannot.