Astigmatism is an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness.
Why does vision worsen as we age?
As we grow older, the lens becomes less elastic. It loses its ability to change shape. This is called presbyopia and is more noticeable when we try to see things that are close up, because the ciliary body must contract to make the lens thicker. The loss of elasticity prevents the lens from becoming thicker. As a result, we lose the ability to focus on close objects.
At first, people begin holding things farther away in order to see them in focus. This usually becomes noticeable when we reach our mid-forties. Eventually, the lens is unable to move and becomes more or less permanently focused at a fixed distance (which is different for each person).
To correct this, bifocals are required. Bifocals are a combination of a lower lens for close vision (reading) and an upper lens for distance vision.