We see things because light reflects off of them and enters our eyes. The light passes through the pupil and into the lens of the eye. The lens is filled with a dense, clear gel which slows the light rays down and causes them to be refracted, or bent. The refracted light rays are then focused onto the retina, a nerve-rich layer in the back of the eyeball. When the cells of the retina are struck by light, they undergo chemical changes that can be interpreted in the occipital lobe of the brain, which then processes this information to create the sense of vision.
A person who is near sighted is suffering from myopia, a condition in which the image is focused in front of the retina rather than on it. A nearsighted person can see things that are close to them, but things at a distance appear out of focus. This can be corrected with eyeglass, contact lenses, or with laser surgery.