Fortitude is a saving throw in Pathfinder: Wrath of the Righteous.
Your fortitude reflect your ability to stand up to physical punishement or attacks against your vitality and health.
Generally, when you are subject to an unusual or magical attack, you get a saving throw to avoid or reduce the effect. Like an attack roll, a saving throw is a d20 roll plus a bonus based on your class and level, and an associated ability. Your Constitution modifier is added to your Fortitude saving throws.