The Absorption modifier completely erases a certain amount of damage, usually of a specific type, as well as heals the user that amount of damage before they take damage from the rest of the hit.
The value of absorption modifiers varies greatly due to how damage is inflicted:
Absorbing lightning damage is the most useful since lightning tends to hit in multiple small bolts. Ten points of lightning absorb can completely neuter a Lightning Enchanted boss, if it's emitting dozens of 6-8 damage charges. In contrast, ten points of fire absorb would make hardly any difference against 500 damage Meteors, this also applies to cold damage.
Absorb Magic Damage modifiers work against all types of non-physical damage.
Absorb Damage modifiers are very useful against physical attacks, especially multiple small damage hits, such as those from Flayer blow darts or Skeletal Archers.
Absorb X% Damage modifiers are most useful against fierce melee damage monsters such as Hell Bovine or Moon Lords, who hit for massive physical damage.