These work like a room projector, which uses a bright lamp to send light through a small LCD, which is then magnified through a lens onto the screen.
In a standalone projector, the projector and lens is inside the base of the set, and the image reaches the back of the screen through a system of mirrors. The screen itself is just translucent plastic.
Major problems here are bulb wear out, and bulbs are expensive, and dust on the mirrors and other internal optical surfaces.
If you have one it is still potentially useful, but don't pay money for it.