An Autostereogram is an image that can be percieved as 3D when viewed properly. Repeating patterns can make your eyes focus on two different things while perceiving them as one, creating the illusion of depth.
To make your own, upload a black and white image and click "Render". White will be percieved as close, black will be far away. To view the image, hold the image close to your face, and cross your eyes so that the two red dots on the top of the image overlap.