A Mana Star is used to permanently increase your maximum amount of Mana.
There are four types of Mana Stars. The Small, Medium, and Large Stars are obtained as Monster drops from different Combat Areas, and give +1, +2, and +3 Maximum Mana, respectively. The Huge Stars are obtained through special means and give +5 Maximum Mana.
Huge Mana Stars add +5 to your maximum Mana and are obtained as one-time special rewards. There are currently only two Huge Stars released, though there will be five in total. The first is received as a reward from The WizardQuest and the second can be received as a reward from the Castle Chest.