Imagine that your eye is like a camera, and the retina is the film.
Rays of light enter the eye and are focused on the retina by the lens.
The retina produces a picture which is sent along the optic nerve for the brain to interpret. It's rather like the film in the camera being developed so that pictures can be produced. Usually the retina is attached to the inner surface of the eye. If there is a tear or hole in the retina then fluid can get underneath it. This weakens the attachment so that the retina becomes detached - rather like wallpaper peeling off a damp wall.