Modern 3D cinema uses a system of polarised light. Light travels in waves that oscillate in different directions and angles. Films in 3D use two projectors, which are each passed through polarising filters that only allow for light travelling in one direction. One projector has a filter that only lets vertical waves of light through, and the other only lets horizontal waves of light through. The naked eye sees light waves in all directions, so without the 3D glasses you can see the output of both projectors as two images at once. Seeing light waves in just two directions cuts down the amount of light that the eye sees, which means that the film is dimmer than conventional 2D films.
Answered by David Houston.