Stereo's or Magic Eye images are popular art forms that arose during the 1990's when brightly colored calendars and posters showed up in stores and shopping malls nearly everywhere.
The word "stereo" comes from the Greek language and literally means "relating to space". Years ago I played with a freeware program that made 3-D images out of artworks, photographs and such but one could only view the resulting images by slightly crossing ones eyes slightly to slowly coax the image into view.
I have no idea where the software went but recently I stumbled upon these three images that I'd made using one or two snowflake patterns from paper snowflakes website. To view these magic eye images, you must focus on the two dots at the top of the image and cross your eyes until the two dots converge.
Keep practicing. It may take a while but eventually the snowflake images will literally jump out of the page in 3-D. The nickname for stereoscopy is 3 D (for three dimensional).
Stereoscopic pictures are pairs of nearly identical photographs of the same identical objects or scenes taken at different angles. The angles correspond with the natural angles of vision for human eyeballs when looking at an object. Stereoscopy viewing is only possible with persons who have binocular vision. (two good, working eyes). Test this yourself by looking at one of the images while covering one eye with your hand.
You can test your binocular vision by looking at an object with both eyes then, one at a time, cover or close one eye and notice the angle of the object, then cover or close the other eye to see the same object at a different angle.