A prologue is an introduction. It gives your readers information they might need in order to understand the action in a story. It’s an explanation for why the story events happen. It can also provide the reader with essential information about the people and places in the story. It can provide the reason for a feud, or the seeds for a war. It can provide motivation for the main character to act. It can be an attention grabber for a reader. It can also be a scene from the past that has an influence on the current story events. Another very good use of a prologue is to show a character looking back to the time of the story events. Those are the only a few of the reasons for a prologue. Each writer needs to evaluate their story to determine whether they need one or not.
That said, you have to decide if you truly need the information. Would it be boring to read? I once dumped four pages of prologue when I realized that, if it were a movie, it would occupy three seconds of film time. Ask yourself if you are doing an info dump. If you are, don’t use it. Then ask yourself if you can work the prologue into the first chapter, if you can, do so. Your goal with a prologue is to give your reader vital information without boring it. The information should be information that can’t be worked into the story elsewhere. If your prologue can be inserted later in the story as a flashback, that might be the better way to go.
The prologue can only do its job if crafted carefully. Keep the info jump down to a minimum. You should ensure that the tone of the prologue matches the tone of the story. In other words, don’t stick a comedy on the front of a tragedy. Don’t be dry in the prologue and more interesting in the story. Well you could, but the reader won’t stick with you. If you can, and you feel you need a prologue, keep it interesting, make the reader want to read the first chapter. Don’t forget to hook the reader again in the first chapter. The prologue should hook the reader to make it read the first chapter, which will set the hook and make the reader want to read the rest of the story. Then go for it. Used correctly, a prologue can enhance the story. If used incorrectly, your story will lose the reader before it even begins. A prologue is a knife edge. It’s best to be careful when using it. You want to make the reader read on, you don’t want to hurt your story.