Stereopsis is one of the processes the eyes and brain use to perceive depth. As we have two front facing eyes, each taking in a slightly different perspective, the brain is able to judge the horizontal differences between correlating objects as they appear in each eye and understand the distance and depth between objects in the field of vision. Stereoscopy takes advantage of this quality of sight through presenting slightly different images at the same time.
Source from my remembered knowledge from my sensations and perceptions class and from wikipedia.
I’m on a gif making spree.