An autostereogram (also known as Magic Eye) is a 2D image designed to create the illusion of 3D. In each image, there is a 3D object that can only be viewed by looking at the image a certain way, as if the screen was transparent and you looked at the wall behind it.
Correctly viewing a Magic Eye definitely takes some practice, and some people are completely unable to see them. This website is a tool to reveal the 3D scene hidden in a stereogram by revealing its silhouette. It works for most of autosterograms, especially if they have a simple plane as background.
See my Magic Eye generator here.