The mana stat is used for excessive use of magic. When the mana bar (MP bar) dries out, you cannot cast magic until you have either waited for the bar to fill up, or you can drink an MP potion. Potions are the most recommended, as they are very cheap and easy to obtain from monsters that drop a lot of potions, such as Akris.
Classes Dependency of MP Edit
Not all classes need mana to fight. A good example would be the Aloken and Azure Knights. These do have extremely useful skills that require mana, though.
Incar Magicians, hence the name "MAGIC-ian", are completely useless without MP. They need a ton of potions, even health potions. This is because when an I.M gets hit by a monster that attacks with physical attacks, such as an archer or fighter, they take a lot of damage due to their physical weakness. They have a better elemental defence than most other classes, though. This makes up for their weak armors.
The class that can fight without mana at all is the Azure Knight. They look at magic as cowardly and would rather fight close up. They have a skill called Strengthen Health, which refills their HP at a rather quick rate. Because of this, they rarely need health potions, except for in boss battles and tougher fights.