It indicates that something happens earlier in time or comes before something else.
Here are some examples:
* Pre-game: Happening before a game.
* Prehistoric: Relating to the time before recorded history.
* Premature: Happening before the expected or normal time.
* Premonition: A feeling that something is going to happen before it does.
* Preface: An introductory part of a book that comes before the main text.