What is a Prologue:

In literature most stories have an introductory plot. This introductory plot tells how the story began by laying out some details in the form of a separate story or in the beginning of a book. This introductory story may explain how a child was born with gifts and explain how he or she got those gifts. When a writer goes in depth on how the story began, in reference to a story, screenplay, or theatrical performance, it is called the prologue. Some  writers will put the prologue within the story, depending on their fashion of writing or revealing the details of the plot.