Your eye works just like a little camera. The rays of light enter your eye through the cornea which is the front clear window of the eye. Then the lens behind the pupil focuses the light on the retina the equivalent of the film in the camera.
A cataract is a clouding of the natural lens inside your eye. Objects become blurry, bright colors become dull and driving at night becomes more difficult due to glare and halos.
The only treatment available for cataracts is surgery. The old cloudy lens is removed and replaced with a new, clear artificial lens.