Guard
Typically used with: Endurance.
In short: take a hit. If Dodge gets you out of the way of damage, Guard's your buddy once you've screwed that up.
If you fail a Guard Roll, the amount by which you miss the mark is the amount of HP you lose. A Critical Failure of a Guard Roll actually causes harm to any armor you're wearing. See the Combat chapter for more information.
Difficulty
Most of the time, a Guard Roll is used against an incoming attack by a moving opponent. However, there are nasty obstacles out there that can cause a character harm, too. Traps and other damage-causing hazards have a fixed Damage DL. A very well-equipped and tough hero might be able to fall in a pit trap and have the metal spikes below clang harmlessly off her armor.
Opposed Rolls
Guard Rolls are used against another character's Damage Roll or Casting Roll.