I had a little time to do some research. Here is what I found:

Apparently Level, HP, MP and EXP are concepts originally associated with Role Playing Games. There they measured the level (LEVEL) of your characters progress along with the amount of health points (HP) and magic points (MP) and the experience (EXP) your character had accumulated.

For forum usage:

Level is based on how many posts the user has made. Only one post is required to advance from level 0 to level 1. A mathematical formula determines how many additional posts are necessary to advance to the other levels.

HP represents how active the user is by dividing the number of users posts per day by a level of posts per day set by the administrator.

MP represents how much you have posted for the amount of time you have been registered. It's a complicated formula.

EXP is a percentage showing how many more posts the user has to make to get to the next level. When you achieve 100%, you advance to the next level and the EXP is reset to zero. The rate of how fast this increases is customized by the administrator.

The administrator can turn this entire feature off if they chose to do so.