Normally, light is focused by the cornea and lens to form a sharp image on the retina.  Sometimes the eyeball is too long for the shape of the cornea, so the light comes to a focus before it reaches the retina. 

As a result, objects in the distance are blurred although close objects are usually clear.  Short- sightedness can be corrected by spectacles or contact lenses. 

These lenses correct the vision so that distant objects are clear once again.