A mana point is Oberin's measurement for magical ability, basically each spell requires a certain amount of mana points, if your character does not have enough mana points to cast the spell it will not work. This also applies to both mixing potions with Alchemy, and to Wizards using their Enchanting skill to extract etherite and to infuse crystals, because each potion and each Enchanting item requires a certain amount of mana to make. A character's mana point capacity increases as their Class Level increases.