Mana is a primary resource in Paragon. Most heroes in Paragon require Mana in order to use abilities. If a Hero doesn't have enough Mana to use an ability, then the ability will be unavailable to cast.
Each hero has a maximum amount of mana they can have at a time. Every time a hero levels up, that maximum amount also increases, along with their current mana.
Each Hero regenerates mana over time. Mana Regeneration is a stat that can be changed. A high mana regeneration causes mana to regenerate quickly, while a low mana regeneration causes it to regenerate slowly. Usually, it's a rather low number, but gems and cards can be used to regenerate mana faster. While in the hero spawn, a hero's mana regeneration is increased dramatically, allowing them to max out on their mana in a matter of seconds.
Unlike Healing and Health Regen, Mana Regen has both flat and percentage increases. Percentage increases are always based on your Hero's Base Mana Regen, and will not be affected by any flat increases from abilities or Gems.
The formula governing Mana Regen is:
Mana Regen = Base Regen x (1 + Percent Regen) + Flat Regen
To show an example: if your Hero's Base Mana Regen was 3, you were at 50% Mana with the Mana Fountain Gem, and had 120% Mana Regen built it would look like this:
3 x (1 + 1.2) + 5 = 11.6