This is the famous “checkershadow illusion,” invented by Prof. Edward Adelson. A very thick green checker is sitting on a checkerboard. Pay close attention to the two squares of the board that are marked "A" and "B". Believe it or not, these two squares are precisely the same shade of grey. You can verify this by blocking off every other part of the image using a piece of paper with two holes cut into it, or you can just click on the image itself for an animation, or you can check out one of the other interactive versions of the image found in various places on the internet, such as

For a very interesting variant of the Adelson contrast illusion, click here. And see two variations, one using colors rather than shades of grey, here.