Mana is a form of energy that is used for using abilities. There are 6 types of mana: fire, earth, air, water, light, and shadow. It exists within people's bodies, and it flows through the world similar to wind/water currents. For example, you can find high concentrations of fire mana near volcanoes, lava lakes, etc.
The higher level someone is, the more infused their body would be with a type of mana (In noob speak, the higher the level the more the mana), the type typically depending on the race and class of that person, as well regenerating mana faster. The more mana one uses, the more tired they become, and once they hit zero or less mana, they collapse (OOM). There is one mana bar in the VE, which shows how much mana a person has left, though a person would likely be able to "feel" how much mana they have left without it.
Mana regeneration does not only depend on the stats of the person, but the surroundings as well. An area that has high concentrations of a type of mana will have an increase in regeneration in that type of mana, while a lack of it decreases regeneration. During the month of Manaflow (refer to Aetherian Calendar), all types of mana regen are increased. There are also potions and items that can increase mana and mana regen (it was mentioned briefly in NK6).
Mana is a requirement for many abilities. If you attempt to use an ability without enough mana, you will collapse (whether the ability is successfully casted before you collapse is still uncertain). How "Channel" abilities work is still uncertain. Passive abilities do not consume mana.
Devotion (refer to Attributes and Affinities) affects mana, the amount of mana you have, and how fast the mana regenerates.