MP, short for Magic Points or Mana Points, is one of the three basic forms of energy a character has. All spells cost a certain amount of MP to cast, making it very important for magic users to keep their MP full as possible. It can be restored with Refreshers.
Players have a maximum value for their MP. This value is a determined by their INT, and some classes have a lower MP to INT ratio than others. The maximum MP a player can have can also be raised by buffs and awakened or statted equipment.