I think of Shakespeare's quote: "the past is prologue."
That then means, to me, the prologue is going to encapsulate some history--right there before the story begins. A kind of backstory really to set the mood, the tone--or to prepare the reader in some way for what is to follow.
You gave me the answer I needed! Thank you so much!
I met with Alton Gansky at a writer's conference a few years ago and he suggested I add a prologue for the following reason. Chapter 1 was all about the villain and what was happening drove the rest of the story. He said Chapter 1 should be about the main character, so in order to do that my original Chapter 1 had to be moved to the prologue and then the main character started Chapter 1.
Students in my classes have learned that the prologue can be critical. Before coming to my class, many thought that it was something to skip. Then they quickly realized that the author intended for them to read the prologue as part of the story.