Doctors and other health care professionals use several models to help explain and monitor chronic problems. One popular model is the Nagi model.
In the Nagai model impairment refers to the loss of normal body function. If the neck doesn't move, or the kidneys don't make urine, or the heart doesn't beat normally, there is an impairment.
Disability is any restriction or lack of ability to perform an activity normally. The disability shows how the impairment affects a person at home or at work. A person may have heart disease but there's no disability because he or she can still do everything normally. In that case there's an impairment but no disability.
In the case of neck pain, you may have limited range of motion but can still bathe using a bath brush and shoulder motion. The neck is impaired but you aren't disabled.
Not all disease leads to impairment. Likewise not all impairment leads to disability. Doctors and therapists must look at both as part of your overall exam and treatment plan.