After killing an enemy in the Pit, the Protagonist can try and grab the pick-up that falls out. Every pick-up yields experience.

There is an experience meter is in the upper righthand corner of the HUD.

Every time you gather 100 experience, a level-up line is spawned a bit further down the pit.

Extra experience meters will pop-up if you get more than 100 experience before hitting a level-up line.

As you gain more levels, the minimum and maximum ammount of experience a pick-up can give is lowered.