This is pretty simple. A story is started, and each poster continues the story from the previous posts. Be as serious or as silly as you like. Try to keep your posts to one or two sentences if you can, and please don't leave partial sentences for the next person to finish. Other than that, have fun with it!
I will start the story....
The year was 2071, and a little anime catgirl was born. Her parents named her Lucy.