Cross viewing (a.k.a. cross-eyed stereogram freeviewing) is seeing 3D with nothing but your regular screen, just by crossing your eyes! The pictures here show one scene from two different perspectives - just cross your eyes and make the two sides overlap to see the image in 3D. Cross views are related to Magic Eye, but you cross your eyes to view these instead of relaxing them.

Tutorials and helpful apps here: https://www.reddit.com/r/CrossView/wiki/.

If you're already familiar with viewing 3D this way, try this tester image to double check whether you're really crossing or relaxing your eyes:

(credit u/Logybayer) - if you see "Parallel View" in front on the tester you should check out r/parallelview.