Resistance is an entity value that reduces the dice level of incoming damage, usually of a specific damage type. When an entity is subject to damage dice of a type for which it has Resistance, the damage dice level is reduced by an amount equivalent to the corresponding Resistance value.
For example, if an entity with Fire Resistance 2 is subject to 2d8 Fire damage, the roll is modified to 2d4 instead.
Resistance applies to any sources of damage, not just those in a formal Damage entry, and it applies separately to base and enhancement dice for any such roll.
If an entity has Resistance to the same type from multiple sources, it should use the largest Resistance value; the others have no effect.
Resistance values may also be modified up or down from their current value. When this happens, the modifying number is written before the Resistance type and preceded by either a + (for increase) or - (for decrease). Example text might read “the target gains +1 Fire Resistance” or similar. In this case, you take the target's current Fire Resistance value and add 1 to it.
Resistance values cannot be reduced below 0 by effects that decrease Resistances. For instance, an entity with Fire Resistance 1 that receives a -2 Fire Resistance effect would have an effective Fire Resistance 0. However, if the entity received another +1 Fire Resistance effect during the same duration of the previous effect, they would still have the same result of Fire Resistance 0, since the full -2 modifier is still being applied.
The opposite of Resistance is Weakness. An entity that has both Resistance and Weakness should only use the larger of the two, but subtract the difference. For example, an entity with Fire Resistance 2 and Fire Weakness 1 would apply a result of Fire Resistance 1. An entity with the same value for both Resistance and Weakness would thus have them cancel out and apply no modifiers to the corresponding damage type.