Well, there's a very involved description somewhere in the forums, but basically, it's just a mod Strolen thought would be fun. HP is based on how many days you make your post-per-day-quota, which I think is set at 1. MP is based on how many days you've been a member. And EXP is the number of posts you have to make to go up a level.
Welcome to the Citadel, by the way.