In order for us to see, the eye needs to focus light, similar to a camera. Let's break it down:
The cornea and lens of your eye is similar to the lens in a camera. These structures, located on the front of the eye (cornea) and just behind it (lens), bend light rays so that they focus on the back of the eye.
Ideally, these light rays will focus directly on the retina, a very thin tissue that lines the very back of the eye. The retina is similar to the film of a camera. The cells on the retina interpret light rays and send signals to the brain, and this allows us to visualize the world.
If light rays do NOT focus on the retina, either in front or behind it, then that means you have a refractive error or eyeglass prescription. Glasses and contacts help bend light rays so they land on the retina, giving you clear vision.