Our ears use two different ways to determine direction of a source: the difference in when the sound hits each ear (more for low frequencies) and the distortion that your head causes as sound passes through it to the far ear, usually just a lower intensity (more for high frequencies). Similarly, our eyes have two methods to determine the distance of a source (direction is basic): the direction which your eyes point (seem to be more useful for things far away) as well as how your eye needs to focus (very useful for close objects where only one eye can be used).
The common method to do 3D film is to give each eye a different image, with a separation between objects to make them seem further/closer than the screen. If you sit in the right place (as centered and back as possible) the illusion is pretty satisfactory, especially if the director opts out of the eye-catching/suspension-of-disbelief-rattling idea of having things seem to come out at you. This completely disregards the focus method to determine depth, which doesn’t really matter if the objects are far enough away and the director doesn’t apply any shallow depth of field effects (bokeh).
Commentary aside, this is a picture of my dorm from ’05, just took two pictures about the same separation of our eyes and combined the two (switching lef-right). Classic simplicity. .To view, use any of those tricks you use to cross your eyes: hold up a finger somewhere around 1/2 to 3/4 of the way to your screen and look at that until the background matches up, or look really close and move backward while keeping your eyes steady, or just cross your eyes- I think that is easiest.
By the way: to make a true-3d image (which you could focus on) you could make an array of ridiculous resolution screens, each only having a small pin-hole to emit light a few mm away. This will create a “light-field” display which would (almost completely) project light position AND direction.
~and yes, we did clean our room every once in a while.