The eye is an optical system that focuses light on the retina. The cornea is the transparent "window" at the front of the eye. It provides most of the eye’s optical focusing power. If the optical system of the eye works just right, then the image formed on the retina is sharp, and the eye should be able to see "20/20" vision without the need for glasses.
On the other hand, if the optical system of the eye is imperfect, the image formed on the retina is not sharp, and the result is blurred vision. By far the most common conditions causing this are myopia, hyperopia, or astigmatism. These are termed refractive errors of the eye,
Refractive surgery is any surgical procedure aimed at correcting a refractive error.