Generally, when a character is subject to an unusual or magical attack, he or she gets a
saving throw to avoid or reduce the effect. A
saving throw is a 1d20 roll plus a bonus based on the character's class and level (the character's base save bonus) and an
ability modifier.
A natural 1 (the d20 comes up 1) on a
saving throw is always a failure. A natural 20 (the d20 comes up 20) is always a success.
Fortitude: These saves measure a character's ability to stand up to massive physical punishment or attacks against his or her vitality and health such as
poison and paralysis. Apply a character's Constitution modifier to his or her Fortitude
saving throws.
Reflex: These saves test a character's ability to
dodge massive attacks such as explosions or car wrecks. (Often, when
damage is inevitable, a character gets to make a Reflex save to take only half
damage.) Apply the character's Dexterity modifier to his or her Reflex
saving throws.
Will: These saves reflect a character's resistance to mental influence and domination as well as to many magical effects. Apply the character's Wisdom modifier to his or her Will
saving throws.