Mana (MP) is the primary resource used by priests and mages to summon their spells. Every spell uses a certain amount of mana, which is used when the spell is first cast. Mana starts full, and regenerates over time.
The mana regeneration speed depends on combat, whether the character is sitting or laying down. Intellect increases the maximum mana, as well as the speed mana regenerates. Some equipment also has stats that directly boost mana regeneration.
Increasing Your Mana
Your base mana will increase as you level up in the game. Acquiring Guardian Stones by clearing dungeons with a group is a good way of passively boosting your maximum health, mana, fury and endurance stats, as long as your character meets the minimum level requirement of that guardian stone.
Having more Intellect will increase your total mana. Mages yield more mana per point of intellect than Priests, but healing-oriented Priests will have more intellect than Mages. This is seen on equipment where Mages have to focus on more Magic-focused equipment than equipment that possesses more intellect.