Weakness
Source Player Core pg. 400
If you have a weakness to a certain type of damage or damage from a certain source, that type of damage is extra effective against you. Whenever you would take that type of damage, increase the damage you take by the value of the weakness. For instance, if you are dealt 2d6 fire damage and have weakness 5 to fire, you take 2d6+5 fire damage.
If you have a weakness to something that doesn't normally deal damage, such as water, you take damage equal to the weakness value when touched or affected by it. If more than one weakness would apply to the same instance of damage, use only the highest applicable weakness value. This usually only happens when a creature is weak to both a type of damage and a material or trait, such as a plasma doshko critically hitting a monster that has weakness to fire and electricity.
If you have a weakness to something that doesn't normally deal damage, such as water, you take damage equal to the weakness value when touched or affected by it. If more than one weakness would apply to the same instance of damage, use only the highest applicable weakness value. This usually only happens when a creature is weak to both a type of damage and a material or trait, such as a plasma doshko critically hitting a monster that has weakness to fire and electricity.