The idea, originally made by Nisovin (aka that lords of minecraft guy) for a school writing prompt, is to make a story based on the given option, setting, and characters, in the given time. There are rules on the website telling you how you should write the story.
I think this could be a cool idea to get the story forum more active, and to also just get our writing minds going, because I know for sure some of us (like me) aren't that much of a fan of writing. I've already written one story for my english class for extra credit during my study hall, and it is really pretty fun.