Accommodation is really nothing more than the eye continually focusing and refocusing on the objects around you as you change position. The lens becomes flatter or rounder, longer or shorter, depending on the distance of the object in focus. Some people, however, have poor accommodation, meaning that the time between you moving and the refocusing on the eye is too long and vision can get blurry. A small lag is normal and not noticeable to the average person.
The main symptom of an accommodative anamoly is eye strain and blurred vision, though irritation, headache, and vertigo can also be signs of an accommodation problem.
Study is still being done, but accommodation seems to be a result of over-rigidity of the lens or weakened ciliary muscles, the muscles that cause the expansion and contraction of the lens. The causes of this condition remain hazy if it is a rigid lens that causes the problem, but usually a weakening of the ciliary muscles is a response to some other condition in the body that can vary from the flu to nasal obstruction to anemia or high blood pressure. To treat an accommodation anomaly it is best to treat the cause; glasses for eye strain or removal of nasal obstructions if there are any, and so on. Most people, regardless of the cause of their problem, will receive reading glasses, and some will also be given the option of doing reading exercises.
Evans, Bruce J. Binocular Vision.