A Retinal detachment is a disorder of the eye in which the retina peels away form its underlying layer of support tissue. Initial detachments may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blindness. It is a medical emergency. The retina is a thin layer of light-sensitive tissue on the back wall of the eye. The optical system of the eye focuses light on the retina much like light is focused on the film in a camera. The retina translates that focused image into neural impulses and sends them to the brain via the optic nerve. Occasionally, posterior vitreous detachments, injury or trauma to the eye or head may cause a small tear in the retina. The tear allows vitreous fluid to seep through it under the retina, and peels it away like a bubble in wallpaper.
Here is a YouTube link to a video about a retinal detachment.