Base mana refers to how much mana a character has before it is modified by talents, intellect, enchants, and other item effects. As of patch 3.0.2, all spell costs are a percentage of base mana. Prior to that, most spells cost a fixed amount of mana, rather than a percentage of base mana, with a few exceptions. The value is dependent on class and race, only if you are gnome (5% more BaseRecurses)