A round robin means that one author starts the story and other authors can contribute to it. For instance you'd write the first chapter, I could write the 2nd chapter, someone else could write the third... Whoever who seems interested can write a chapter.
I think though that most of the authors who have the round robin icon either don't understand what it is or have it checked by accident. Cuz I'm pretty sure most authors are writing their stories chapter by chapter themselves.
