The shape of your eye determines how well your vision can focus. Light rays enter the eye through the clear cornea, then through the pupil and the lens.
In a normal eye the light rays are focused onto the retina, a light-sensitive tissue lining the back of the eye. Signals from the retina are sent through the optic nerve to the brain, where they are interpreted as the images we see.
Refractive errors exist because the curvature of the cornea doesn't match the length of the eye so light rays cannot focus properly on the retina. Refractive surgery procedures attempt to fix this by changing the shape of the front surface (the cornea) or adding a permanent lens to improve the focus.
Eye conditions treatable by refractive surgery include: