These represent ususual limitations or restrictions placed on a character. These may be due to past injury or mental outlook.
A character either has a particular disability or does not (there are no scores or gradings in disabilities).
Non-human characters will normally have several of these disabilities based on their race (racial disabilities). Otherwise, only a few people in the general populace will have any particular disability.
An example of a disability is Blindness, which means that a character cannot see.
A character's disabilities will usually remain constant during later play; characters cannot normally "lose" disabilities (except through powerful magic).