A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. The lens works much like a camera lens, focusing light onto the retina at the back of the eye. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away.
The lens is mostly made up of water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it. But as we age, some of he protein may clump together and start to cloud a small area of the lens. This is cataract, and over time, it may grow larger and cloud more of the lens, making it harder to see.