Depth on a Flat Screen
A 3D movie can make objects seem to leap out of the screen or stretch far back into the distance. Yet the cinema screen itself is completely flat. So how can a flat surface create such a strong feeling of depth? The answer lies not in the screen, but in a clever trick that copies the way human eyes naturally see the world in three dimensions.
How We See Depth Normally
In everyday life, our sense of depth comes partly from having two eyes. Your eyes are set a small distance apart, so each one sees the world from a slightly different angle. The left eye and the right eye receive two slightly different views of the same scene. Your brain takes these two images and combines them into a single picture, and the small differences between them are what give you a vivid sense of depth and distance. This is called stereoscopic vision.
Two Images, One Scene
3D movies copy this process. Instead of filming with one camera, a 3D film is made using two lenses or cameras placed side by side, a little apart, just like a pair of eyes. They record two slightly different versions of every scene. When the movie is shown, both of these images are put on the screen at the same time. The challenge is then to make sure each of your eyes sees only the image meant for it, and not the other one.
The Job of the Glasses
This is where the special 3D glasses come in. The glasses are designed so that each lens filters the light differently. One lens lets through only the image intended for the left eye, while the other lens lets through only the image intended for the right eye. Some systems do this using differently colored filters, others using polarized filters. Whatever the method, the result is the same: each eye receives its own separate image, your brain merges the two, and the flat screen suddenly appears to have real depth.
Source
This article was written using information from Wikipedia.