A term from Polynesian culture, Mana is the concept of a life force or power in the general sense. The concept of mana is a staple of a lot of RPG's. Mana is usually a stat, a renewable one like HP, used to cast magic spells. The term MP usually stands for Mana Points or Magic Points. Mana, in games, is basically a magical life-giving energy. Items, like Ether typically refill a characters mana. The image of a "Mana tree" that gives the world mana is common.
Mana may have other makes, such as Magicka in The Elder Scrolls V: Skyrim.