McMMO's main function is to incorporate an RPG-like skill system that players will have access to right from the beginning. Depending on how much a certain tool is used or a certain action is performed, players will gain experience that will eventually add bonuses such as double drops, rare drops, or ability enhancement. Currently, there are only three groups of skills: Gathering Based, Combat Based, and Miscellaneous.
Skills you can train are listen bellow.
For more information about MCmmo I suggest you check out their own wikia.