A myth is a story that’s told again and again and serves to explain why something is the way it is. A creation myth, for example, is a story that tells how the world came into being.
You may have studied Greek or Roman myths in which gods and goddesses wage war and play tricks on each other. These myths are not necessarily true stories from the past — the main idea is that they explain certain ideas about the world and how people act. The story might be accepted as true and serve to explain some fact about the world, or it might be known to be made-up but nevertheless illustrates something about people or history.