One of the common pieces of advices heard around writing group and seminars is don’t writer prologues.
Prologues are short bits of narrative places before the opening scenes of the novel and there is great debate about their utility. In my opinion there are time when a prologue is required if you want the reader to fully understand the nature of your piece. Like SF Author Nancy Kress I think there is an implicit promise made to the reader at the start of any story and if you violate that promise you are likely to have an upset reader. The prologue can help ensure that the tone and expectations of the piece are well established before the meat of the plot has begun to unfold.
Think about the book, and even the television series, A Game of Thrones. Both start with a prologue where we follow a small band of ranger north of the wall encountering the first stirrings of the army of the dead. Except for the execution scene in chapter one we do not see these characters again. They play no major part in the plot and have no deep familial connections to any of the main characters, but this prologue is essential. Without that prologue we would not know that this is a world of magic and zombies. We are well aware of the danger that so many characters will spend so much time dismissing.
Too often this is not how writers try to use a prologue, but rather they often take a short cut into one of two broad uses.
World-Building;
The prologue will try to lay out too much of the history of the world, to give too much context for the viewer or reader and in the end bore them with a history lesson without emotional meaning. This is very tempting and it should be avoided. The real craft of world-building is laying the foundations just before you actually need them. Sprinkle them throughout the work in pieces to fall into place as the narrative unfolds both enlightening the reader and explaining the world but without stopping to lecture. This is is very difficult to do and so many people try to take a short cut and dump this stuff in the most critical place in the book, the beginning.
A Fake Action Opening:
Attempting to follow the advice to always open with action, writers will craft a battle oriented prologue, something composed action, daring, and high stakes, but all too often that is followed by a lack luster first chapter. Many times this will be the inverse of the World-Building prologue because the first chapter will be extensive world-building and the author has hoped that the prologue and its action will carry the reader through the tedious establishment and info-dumps of a poor first chapter.
In both of these case the prologue is a bandage on the more serious issue, either a lack luster opening or less than stellar world-building. The answer is not a prologue but fixing the root cause of the trouble.
There are good causes for prologue but make sure yours is not trying to fix a more fundamental trouble.