Mana is the currency used in Dungeon Defenders, so it is very important that you have as much as possible. It is needed to build new Towers and upgrade/repair already built ones, as well as buy new pieces of equipment and upgrade them. You also need Mana to heal yourself and activate your special abilities.
There are a few ways you can get Mana in Dungeon Defenders. One way is to open chests scattered throughout the stages , which hold Mana Crystals. Monsters, when killed, also drop these, so make sure you pick them up (they are attracted to your character, so you don't need to pick up each one). Also, chests and enemies will sometimes yield equipment, which can be picked up and sold for Mana. Note that any equipment and Mana Crystals left on the ground when the next Combat Phase starts, it's all converted to Mana and placed in your stored bank (which all characters can access on your system).
As you will undoubtedly notice, every character can only hold so much Mana while playing on a stage. This is affected by their level, so the higher it is, the more Mana they can hold. Specifically, the formula for how much Mana is as follows; (Hero's level x 20) + 20. Thus, a level 50 character would be able to hold 1,020 Mana on them. Once you have reached the maximum, Mana Crystals won't be attracted to you anymore until you go below that max again.
