Item traits
In addition to unlocking perks, some items also have traits that can be either good or bad (or both). Traits are inherent characteristics of a weapon or item, and does not depend on skill.
Each armor has a number of so-called armor traits. These give positive as well as negative effects to it's user. The power armor, for example, has the "Tin Can" trait which reduces the chance to be knocked out, but if a knockout nevertheless should happen, it will last longer. The armor traits are also listed in the pipboy so you can check what they mean at any time.