A 3D movie is a film that provides the illusion of a three-dimensional image on a two-dimensional screen. When you go to see a 3D movie, you are usually given a pair of cardboard glasses -- one eye of the glasses is tinted red, and the other is tinted green or blue. When you watch the movie, it'll look blurry and badly-colored if you don't wear the glasses, but when you put the glasses on, the colors clear up, and it looks like everything on screen is coming right at you.
Here's how it worked back in the olden days: Our eyes are separated by a distance of about 2.5 inches, which allows us to see objects from two slightly different points of view and allows our brains to accurately judge distances as far away as 25 feet. To make a traditional 3D movie, you need two cameras whose lenses are spaced 2.5 inches apart, like our eyes. When you develop the film, the film on the left should be colored blue-green, and the film on the right should be colored red. When the images are superimposed, they'll look blurry, but when you put on 3D glasses, the colored filters in the glasses will decode the picture so your left eye sees only what the left camera lens saw and the right eye sees only the right. Your brain combines the images to see true depth and make it seem real.
Unfortunately, no one has made a decent 3D movie since the 1950s.