Mana is the amount of magical energy a hero or unit has. Mana is like health; when there a unit has no mana, it may not cast spells that require mana. Some abilities, such as Avalanche's Blink, do not require mana, and thus, are not affected by it.
Almost every hero has mana regeneration, which they can use to recover lost, or spent, mana. Through items a hero can boost its mana regeneration and total amount. In Footmen Frenzy heroes can gain mana by going to the center of the map, where there is a mana regeneration aura given off the by Archvault.
Items that boost mana regeneration or amount: