Much like prologues, an epilogue contains an important scene that occurs outside the normal course of the novel. In this case, after the story has ended. Some readers just aren't ready to say goodbye to their favorite characters when the book ends. Sometimes, they want confirmation that the HEA really did come true. Hence, the use of an epilogue, which details events from months, even years into the future.
Some published authors have even written epilogues years after a book has been published and posted them on their Web sites because of reader requests to find out what happened next. Epilogues, much like prologues, should be used sparingly, if at all, and kept to a length of two to three pages.