Many effects will cause a character to suffer damage. When a character takes damage, it is added to any existing damage they currently have. Any effect that heals a character will reduce the total damage they have by the specified amount (down to zero).
There is no upper limit to how much damage a character can have. However, each character has a breakpoint - a threshold after which, suffering more damage is extremely detrimental and can result in falling unconscious, or death.
When a character suffers damage, if their total damage now reaches or exceeds their breakpoint, they become weakened. The weakened condition can stack (a character can suffer from more than one weakened condition). In addition, if they are now weakened, they must immediately make a Steel vs Difficulty +2 check; if they fail, they immediately fall unconscious.
See Conditions for details on all conditions, including weakened, unconscious, and stabilized.