A normal eye functions rather like a camera. The cornea is the clear window at the front of the eye. Along with the natural lens inside the eye the cornea should focus light coming from a distance clearly on to the retina at the back of the eye.
The retina contains millions of tiny receptors which convert light into electrical impulses to be carried by the optic nerve to the brain.
The iris (coloured part of the eye) acts like the aperture of the camera, regulating the amount of light getting in.
An eye in which the light from a distance is not focused correctly on the retina has what we call a refractive error.