Q:
Thoughness or Condition damage
Toughness increases the effectiveness of your heals by meaning that your heals fill a larger portion of your health bar as your take less damage from all direct sources
Toughness DOES NOT affect condition damage and you will take the same amount from bleeds etc.
And yes, most of the moaners that come here saying I got backstabbed for 12k+ have less than 1200 toughness and wonder why they get hit so hard