A cataract is when the lens of the eye gets cloudy. It is a treatable medical condition. Cataracts can cause short-sightedness (myopia), which means you cannot see things in the distance but can see things up close.
If not fixed by surgery, some types of cataracts will cause blindness (which means you cannot see anything at all). In a simple operation, the old lens can be replaced by a new plastic one. Many thousands of people have had this operation.