What is Prologue ?
Prologue is (noun) 1. a piece spoken as the introduction of a play or poem The prologue sets the scene and introduces the main characters. Compare epilogue 2. a preliminary event that leads on to something else The discussions between Foreign Ministers are a prologue to the signing of a full-scale treaty. (NOTE: [all senses] The US spelling is prolog.)
source: Easier English, Student Dictionary Upper Intermediate Level