Unlike Armor Class, mitigation directly impacts damage taken by a character in a very concrete way. The percentage of mitigation is directly taken out of any incoming damage, and reduced by the total amount of mitigation.
For example: Your warrior JoBob is attacking Kerafyrm the Sleeper. Kerafyrm attacks JoBob for 1,000 damage, but JoBob has 40% damage mitigation. JoBob will only get hit for 600 damage.