"Afterword" is like the ending of a book, but with more explanation from the author.
You know how when you finish a book, you reach the last page and it's over? Well, sometimes the author wants to say a little bit more about the book, like why they wrote it or how they came up with the idea. That's where the "afterword" comes in.
It's kind of like a special note from the author to the reader, where they get to share their thoughts and feelings about the book they just wrote. It's not part of the story itself, but it helps you understand more about the book and the author's intentions.
So if you see an "afterword" at the end of a book you've just read, it's like a bonus section where you get to see what the author thinks about the story they just told you.