Mana is the main resource that is needed to use most skills. As long as your mana is above 0, you are able to use a skill even if the mana cost is higher than your remaining mana.

Mana values can be increased through items, passives and by leveling up your character. Each character level grants 0.5 mana.

When using skills that cost more mana than the amount of mana you currently have, you will go to negative mana. You can still regen mana and use skills that do not have a mana cost while at negative mana, however skills that cost even 1 mana cannot be used while at negative mana. Some skills have benefits while being at negative mana.

Your character will also have 8 mana regeneration by default, which can be increased through skills, items and passives, but not by leveling your character up.