A prequel is an installment in a series of books or movies which describes action that occurred in the past, before the original. Monsters University, Batman Begins, and Star Wars: The Phantom Menace are famous prequels.
The word prequel is most often used for movies, along with sequel, the follow-up to the initial story. Filmmakers sometimes like to fill in background information with a prequel, though fans often complain that prequels don't live up to the original (think of the original Star Wars films and their prequels, for example). The earliest known use of prequel described a 1958 installment of a science fiction story, and the word was common by the 1970s.