The higher toughness a monster has, the less damage it takes. A monster with 100% increased toughness takes 50% less damage, a monster with 200% increased toughness takes 67% less damage, and so on.

Negative toughness instead causes the monster to take more damage. A monster with 100% reduced toughness takes 100% more damage, a monster with 200% reduced toughness takes 200% more damage, and so on.

Effectively, each 100% of toughness halves the damage a monster takes, and each -100% toughness doubles the damage a monster takes.