The retina is a thin layer of nerve cells lining the inside of the back of the eye. You need the retina to see properly. It turns the light that enters the eye into an image by sending a message along the optic nerve to the brain.
When this light-sensitive layer becomes separated from the inner wall of the eye, it is called retinal detachment. If this is not treated, it usually leads to blindness in the affected eye. Retinal detachment normally occurs in one eye.
Definition from NHS Choices: Detached retina