The natural lens of the eye is made mostly of water and protein. Sometimes, as the eye ages, some of the proteins undergo changes, causing the eye's lens to become discolored and clouded. This is called a cataract.
When this occurs, light passing through the eye becomes scattered and unfocused. The result is that all objects, near and far, become blurry. The only treatment for a cataract is removal of the natural lens and implantation of an intraocular lens (IOL). This restores the eye's ability to have light pass unobstructed through the retina.