Vision depends on light, either from the sun or from an artificial source such as a light bulb.
Light reflects from objects and enters your eyes through a clear covering, the cornea, over the front of the eye. The cornea does most of the focusing of light onto the retina. Behind the cornea is the lens which does some of the focusing and enables the eye to vary its focus. Thus an image is formed on the back of the eye, on the retina. The most light-sensitive point of the retina, near its center, where sharpest vision is achieved, is called the macula.