These are called autostereogram, auto-stereogram, or single-image stereogram (SIS), or sometimes called 3D arts
Some people can see it easily. Some people can't. Some people say the way is to relax and unfocus your eyes and when you see it, you will know.
The word "3D"
Also the word "3D"
Candy
Capsule
Dino
Dolphins
Flower
Dots
Shark
Horse
Find some other autostereogram 3D art on the web