Your Pre-mitigation Damage is the damage of your hits after all your modifiers to damage have been applied, but before the target's mitigation, such as Armour, Resistances or Blocking, prevents any of that damage.

The target's modifiers to Damage taken apply after their mitigation, so Pre-mitigation Damage also does not include the effects of those modifiers. However, modifiers that cause the target to take damage as a different Type occur before mitigating the damage, so are included in Pre-mitigation Damage.