When watching movies in 3-D, viewers wear specialty glasses this gives the movie an appearance of height, width, and depth (3 dimensions). Normal movies, TVs, and videos are two-dimensional (width and height). The enhanced depth perception of 3-D makes images seem more lifelike.
The technology behind 3-D or stereoscopic is pretty simple, it takes back to 1832. The production of the stereograph entailed making two images of the same object with a camera with two lenses placed about 2 1/2 inches (6cm) apart to simulate the position of the human eyes, and then mounting the positive prints side-by-side laterally on a stiff back. Then viewed through a stereoscope. The stereoscope has to eyepieces through which the stereo images, placed in the holder in front of the lenses were viewed. The two images were brought together by the brain to create the illusion of three dimensions. In a movie theater, the reason for the 3-D glasses is to feed different images into your eyes. How 3-D glasses work depends on how the eyes work and communicate with the brain. Humans have binocular vision that works best when both eyes work simultaneously. Binocular vision gives depth perception and allows you to tell which objects in your line of sight are closer or farther away.
If you look at a 3-D image without 3-D glasses. Two outlines extend from the screen one is blue and the other is red and they are slightly offset. With the glasses back on, the brain merges those images to create the perception of depth. The lenses of the glasses control what each eye is seeing by filtering the light going to each eye only allowing certain wavelengths to pass through.