Characters can suffer damage, which represents trauma and injury to the character's physical wellbeing. Damage can come from a variety of effects, although most commonly from the Attack or Strike actions (see Weapons and Strength), or from various talents that grant unique actions.
In any case, when an effect causes damage to a character, the following steps are taken:
Whenever a character gains a weakened condition as a result of damage, they must make a Fortitude check. The character checks their Toughness vs DV10; if they fail, they immediately fall unconscious. (Note that as Toughness is not a skill, it cannot have training or mastery; the weakened condition(s) still apply downgrades, however.)
Be aware that weakened is a stacking effect; the first time a character suffers total damage that equals or exceeds their breakpoint, they gain a a weakened condition and must make a Fortitude check. The character must also do this each subsequent time they suffer damage while their total is equal or above their breakpoint.
There are three types of damage that can be applied; regardless of type, they are still treated in the same way as above. They have several thematic differences (unrelated to the mechanics of the game), and in some cases rules variations. They are: physical, poison, and magic.
Physical damage represents trauma injury, such as impacts, cuts, burns, etc. This damage is by far the most common - weapons (and therefore the Attack action), as well as the Strike action, all naturally deal physical damage. Many talent actions do as well. Physical damage allows for the highest single damage effects, but has no additional effects naturally. Physical damage is almost always defended by either Fortitude or Reaction.
Poison damage is injury caused by poisons, as well as venoms or other ingested, injected, or absorbed substances. Poison damage ignores all armor, but often deals less damage than physical sources, and usually over time. Many sources of poison damage also apply the poisoned condition as well.
Magic damage is injury caused by a magical effect that is not manifested directly as physical trauma. Magic damage often comes with additional side effects as specified. Magic damage is almost always defended by Spirit.
The keyword heal specifically refers to the removal of damage (not conditions). Healing can bring a character's total damage under their breakpoint, or possibly down to 0. A character is no longer weakened if their damage is less than their breakpoint.