Asaalam Alaikum.
Here is a fantastical world accessed only by basement closet doors where upon you can enter many couple hundred fantasy worlds, if I'm to be stingy. The basic idea: you contribute to the progress of a world and its story through your character or what powers you've been designated through writing a post. This can either be one line, or effectively writing an entire chapter worth of content from the perspective of your character, characters, or your power that be; depending on genre.
Certain areas have certain expectations, and in those areas you'll find different genres, and even among certain threads pertaining to that genre you'll have certain rules.
If you're still confused, I advise simply looming over someone and seeing what they do. Or you can do as I did many years ago and dive right in and learn as you go. Of course I dove in through the genre of Nation RP and have become effectively spoiled to the many quarter million possible single-characters I can throw around. There's no shortage of setting or character type to work with.
Just, no dice or stats. But that's my personal politics.