The act of expounding or of laying open the sense or meaning of an author, or a passage; explanation; interpretation; the sense put upon a passage; a law, or the like, by an interpreter; hence, a work containing explanations or interpretations; a commentary.
that part of the structure that sets the scene, introduces and identifies characters, and establishes the situation at the beginning of a story or play. Additional exposition is often scattered throughout the work.
The part of a play that fills in things that have already happened so you can make sense of who's who and why they're doing whatever. Sometimes playwrights use actual narrators to do this. Other times it's cleverly woven into the opening dialogue without you really noticing.