Question

What are the purposes of a prologue in a play?

Answer 1

Its what opens a story and establishes the context which gives the background information. 

Answer 2

To lay out the story's theme and what the play will be about. Such as the characters and what is going on previous to where the play starts.

For example, the prologue of Romeo and Juliet starts off by talking about the feud between families and ends by talking about the death of Romeo and Juliet.