Max Mana is a bonus featured on some items, the amount indicated is the amount of extra mana granted to the user while the item is equipped.

The bonus mana given on equip is added both to the maximum mana capacity and to the current amount, but the games records negatives values of mana (displayed as 0 in the game interface) so unequipping and reequipping Max Mana items won't replenish the mana pool once it has been emptied.