"My armour is home-grown!"
A character with the Exoskeleton mutation has some form of biological armour forming over their body. This could be bone or chitin, but whatever it is it provides significant defence against injury. The skill gives the character one extra hit, which acts as any other basic hit (as opposed to armour), and is healed as normal.
The physreps for Exoskeleton can take many forms, but it must be obvious to anyone who looks at the character that there is widespread mutation. This could be bony plates or hardened scales across the skin, for example.