I'm brand new to this website. Can someone please explain to me what it's all about?

It seemed cool, so I joined. I'm wondering if there's any way to actually write stuff and have AI expand on what you wrote.

Welcome to Notebook! As far as I know, AI doesn't expand on what you wrote, although occasionally andrew (Our Supreme Lord and Overseer) hosts events where you can generate how your characters look with AI, but if you write something then it won't expand, it's not a feature on here. I hope that helps, and I hope you enjoy Notebook!