Mana:

Mana is the resource heroes spend to cast most spells and activate some abilities in Dota 2. A hero’s mana pool is shown as a blue bar, and if it is too low, certain skills cannot be used.

Managing mana matters because spell usage, lane trading, and team fights often depend on having enough mana available at the right time. Mana can be restored through regeneration, items, abilities, and returning to base.