It's a bit complicated to explain, but it's basically:

Take the amount of time you've been here, and see how many posts you've got. If you've made a LOT of posts, even if you've been here a short period of time, then that means you're a "Super Poster". If you've been here a very long time, but don't post very often, then you may NOT be a "Super Poster".

Me, I've got about an average of 8 posts per day since I came here, so I qualify as a "super poster", even though I've not been here every single day