Each element does more damage to one other element, but also receives more damage from a third element. Fire is stronger than Ice. Ice is stronger than Arcane. Arcane is stronger than Fire. Light and Dark cause more damage to each other. You can use items to improve the stats and resistance numbers of your heroes. For example, placing an Ice item on an Ice Hero will increase their attack or resistance against Arcane enemies. Equipping that same item to a Fire or Arcane heroes will not grant the same effect.