Mana is the amount of charge, power, or stamina of a class. Its purpose of the game is to be unable to activate all of your abilities at once as all of your abilities are ready, which will generate unfairness ingame.
As you spawn into the map, you are given your wanted class and will give you 0 mana at first (with the exception of Mechanic). As time continues, the mana bar will slowly generate unless at its maximum capacity.
Mana is very important in battle, as you need a specific amount to activate abilities (with exceptions for some classes) to enhance your strategies and battles. Without them, classes such as Mechanic or Abyss will have trouble surviving from fighters.