Our eyes see the world from two slightly different perspectives. Our brain merges them into a single image that allows spatial perception and determination of distances. The principle of a stereoscopy is to photograph two pictures for both of your eyes.
The classic way to view a stereo image pair is to use a stereoscope.
The profound impression of the stereoscopic effect is hard to grasp if never experienced. The spatial 3D aspect of an image lets you step inside the scene in a very intense and inspiring way.
Stereoscopy was there since the very early days of photography.