This describes a book, movie, TV show, or play that's about something that really happened.
This is different from a documentary, which is a movie that shows a real situation as it's happening. It's also different from a nonfiction book, which is a book that presents facts or discussions instead of a story.
Unlike documentaries and nonfiction books, a movie or book that's "based on a true story" is presented in a story format. Some details of the story may be imagined by the writer or changed from how they originally happened. Only the main idea or plot of the story matches something that happened in real life.