A creature can be Maimed for many reasons, such as taking a critical hit and failing the Fortitude Contesting Roll, or traveling too long, or staying awake on watch for longer than required.
In any case, when a creature is Maimed, it gains a stack of Maimed. Each stack of Maimed has -1 to all Skill Rolls. Typically all creatures can be Maimed. Once a creature reaches 10 stacks of Maimed it is Dead.
This status effect can be removed by a Long Rest, Medicine, Restoration, Adept, or Restoration, Major.
Taking over half your Vitality in a single Damage Roll automatically adds a stack of Maimed.