Stereograms set up flat, still images in such a way that they appear three dimensional to the viewer. In the mid to late 19th century photographers would shoot scenes using a camera equipped with two lenses a few inches apart. This would simulate our binocular vision and, when viewed through a stereoscope, would appear deeper and more lifelike.
The “Stereogranimator” is a tool from the NY Public Library that takes many of these dual images and animates them as looping GIFs. The simple back and forth between the images allow internet users to enjoy the stereoscopic effect without needing an ancient piece of equipment. They give a feeling of looking at a landscape while teetering quickly side to side.
The animations can be a bit jarring as the quickly strobe between the two images. If you really want a head trip check out this page showcasing 30 of the NYPL’s most popular GIFs.
LINK: Hyperallergic has a great article and cool samples.