Stereo animated illusions give you the impression that the image you are seeing is in real 3D space. They are created by taking a picture of object from two different angles, than you combine those pictures into one animated gif by rapidly shifting from one image to the other. Because the perspective on the images is shifting so quickly, your brain interprets those images in 3D.
Enjoy the illusion and don't get a headache by staring at the images too hard.