Damage Reduction
Damage Reduction, also known as Damage Resistance, is a game mechanic popularized by Dungeons & Dragons, although it wasn't formally named as such until 3e. As the name probably indicates, Damage Reduction is a way of handling the various supernatural or logical toughness-related advantages that different creatures may have, resulting in a reduction of damage inflicted, unless the damage was caused by something specifically useful for hurting that creature.
The two most classic examples are skeletons being resistant to non-bludgeoning physical damage and werewolves being super resilient, if not outright immune, to any damage that isn't inflicted by a silver weapon.
Energy Reduction follows the same principle, but specifically applies to one (or, in some creatures, more than one) form of elemental damage. For example, a demon taking less damage from fire, or a yeti taking less damage from ice.