Mana is a game mechanic that comes in two varieties; build mana and hero mana.
Build mana (hereby referred to as simply, "mana") allows heroes to build defenses. Mana is dropped by enemies that are killed; different enemies drop different amounts of mana. Mana comes in different amounts. The current amount of mana your hero currently is holding is listed above your health bar.
The amount of mana a defense costs is indicated on the top of the Defense Tooltip when placing a defense. (The default key/button-bind is "E" on PC and "X" on controllers in order to toggle the Defense Tooltip. This key must be held while a player is placing a defense to toggle the Defense Tooltip. This key/button-bind may be changed in Options > Controls).
The maximum amount of mana a player can hold is 99,999.
Enemies drop mana in the amounts of 1, 5, 10 and 50.
Mana will automatically combine itself into larger amounts whether that mana was dropped by enemies, or players.
Mana will also move towards the player if the player is close enough.
When collecting mana, the amount of the mana you are picking up is shown above your mana total in green text. Each mana piece plays a distinct sound when collected, depending on its amount.
When dropping mana, the amount of mana you are dropping is shown above your mana total in red text.
You can drop mana by clicking the default Drop Mana / Drop All Mana key ("M" key on keyboard, down directional on controller. This key/button-bind may be changed in Options > Controls). Tapping this key/button drops 5 mana (as long as the player has more than 5 mana) at a time; holding this key/button drops all of your mana after a short delay. Mana that is dropped cannot be picked back up by you for 5.5 seconds.
If a player drops all of their mana, the smallest number of individual pieces of mana are dropped. Eg. if a player had 83 mana, the following mana pieces would drop: 50 mana, 10 mana, 10 mana, 10 mana, 1 mana, 1 mana, 1 mana.
Player mana is a replenishable resource every hero has. This resource is used to cast/activate abilities. Every hero has 100 mana, and mana regenerates a little over 2 mana per second. Equipping certain accessories allows you to increase the speed at which you can regenerate mana.