Combatants can have either Resistance or Weakness to each of the eight element types. Resistance reduces damage by 75%, Weakness increases damage by 50% and grants the attacker Tactical Advantage.
Elements:
By default all humans (including Aura) have Weakness to Lightning and Resistance to Light. Non-humans usually have Weakness to Light but this is far from universal. Various skills can grant Resistance, such as Water Skin and Defensive Stance.
Note that when selecting an enemy to attack the interface will display if they have Res Resistance or Weakness to that type of attack, as long as their entry in the bestiary has that information.