In a novel with a typical story arc (an inverted checkmark), there’ll be rising action, a major climax, and a denoument. Sometimes, though, there are still questions that need to be answered about what happens after the story ends.
In the same way a prologue shows what happens before the main story begins, an epilogue is a separate section showing events that happen later. It may jump ahead months or even years, and will give extra insight into the characters’ lives.
Not every story needs an epilogue, but it can be useful to give the story closure. This is fairly common in romance novels, where readers may need additional information about the main characters to satisfy their expectation that they will live happily ever after. For example, it might show the protagonist married and with children. In suspense novels, it can show the outcome of previous events.
Have you used an epilogue? Do you think they add to the story, or do you prefer to have the events that occur after the main story ends left to your imagination?