A round-robin story is like a game where everyone takes a turn adding to a story. Imagine you and your friends are playing this game, and you start by saying, "Once upon a time, there was a magical unicorn." Then your friend adds to the story by saying, "The unicorn lived in a beautiful field of flowers." Then your other friend adds, "But one day, a mean dragon came and started destroying the flowers."
Each person takes a turn to add something to the story until the story is complete. It's like a puzzle, but instead of putting together pieces, you're putting together ideas to make a story. By the end, you'll have a fun and unique story that no one person could have come up with alone.