Detached retina, very simply, is a displacement of the retina from its normal position against the back of the eye.
The retina is a delicate sheet of tissues lying snugly against the back wall of the eye. It functions much like the film in a camera transforming light into a "picture" which in the case of the eye, is transmitted through the optic nerve and "developed" in the brain.
Once the retina is detached, it no longer functions properly in transmitting "pictures" to the brain. The blindness or blind spots caused by the detachment are permanent unless the retina is put back in place.
Retinal detachments are often preceded by holes or tears in the retina. The vitreous, a gel-like substance filling the interior of the eye, will seep under the retina and cause the detachment.
This horizontal section of the eye shows a detached retina. A tear or hole in the retina often precedes a retinal detachment. The vitreous then seeps between the layers of the retina causing blindness that is permanent unless the retina can be reattached to the back wall of the eye.