MCMMO is a plugin that gives minecraft a feeling of MMO games. Wich means you have different skills you can level up and it gives you more bonus the higher your level gets. for example 'Axes' at level 250 you get 'Explosion' wich makes splash damage. with /mcmmo you can find more commands that can help you understand MCMMO.
I hope this anwers your question. If you have more questions let me know :)
Jelly