A cataract is clouding of the lens inside the eye; a condition that is relatively common in old animals. Cloudiness of the cornea (the clear part on the outside of the eye), can happen as the result of injury or infection. Is the eye watery/weeping? Any sort of discharge around the eye? It's possible that there might be some foreign body (a piece of hay, for example) on the inside of the eyelid, causing irritation, or the rabbit's eye might have gotten poked or scratched by something else. Flushing the eye out with saline solution (like is used for contact lenses) can help to wash out things that shouldn't be there, though an infection may need a prescription antibiotic from the vet in the form of eyedrops to clear it up.