It's a variant on the Mutants and Masterminds system.
All weapons have a damage rating. In the statblock, it'll be something like Melee +2 (+2, claws). The melee is your 'to hit' mod, the number in the parentheses is the damage rating. Damage can be Lethal or Nonlethal.
Damage is not rolled, it's just calculated from the damage rating plus any modifiers - usually strength. As an example, let's say a sword has a total damage rating of +3 and the user has a Strength of +3 for a total of +6.
The target makes a Toughness save with a DC of 15 + the Damage Bonus. With the attack above, that would be a 21. If you make it, you take no damage. If you fail, you are Bruised. -1 to further Toughness saves. If you fail by 5 or more, you are Dazed. Fail by 10 or more and you're Staggered. Fail by 15 or more and you're Unconscious. Status worse than Bruised accumulates; a Dazed person who gets another Dazed result goes to Staggered. Further nonlethal damage after Unconscious goes to the Lethal track.
Armor adds to your Defense to keep you from being hit.
So, there are no real hit points; you can just keep going as long as you keep making that Toughness save. In practical terms, you'll blow the roll badly enough much earlier and be knocked out or killed before you get to the point where it's impossible to make a Toughness save.