Wikia

Reflex is a saving throw related to a character's ability to withstand damage thanks to his agility or quick reactions or to prevent snares.

The higher this is, the better you are at dodging attacks such as a wizard’s fireball or the lethal breath of a dragon, and quite importantly to avoid sprung traps. It is used to avoid physical effects that cover an area (e.g. explosions) and losing one's footing. (Spike growth and grease)