It is in the rulebook, last page. The rulebook specifies 3 keywords: guardian, redeploy and ambush.
A character with Guardian can be activated to remove all cards from the opponent's pool showing damage (either melee or ranged). Then you deal that amoung of damage to the guardian.
Essentially you are taking the damage instead of another character.
I know this is an older post, but it was a top result when I searched for it, so I thought clarifying here would be helpful.
Guardian means before that character activates, you can choose to remove one of your opponent's dice showing melee or ranged damage, resolve the damage against your character, and then remove the die. It doesn't apply to all of the opponents damage dice.