You may note the two squares above aren’t exactly the same color; getting a screen grab and compressing the file and all that mucked with the coloring a bit. But clearly, those two squares are very close in shade and color, nowhere near as different as your brain thinks they are in the checkerboard. This classic illusion is due to the way you interpret color (or shading): it’s not done independently; your brain is always comparing things. In one case, the square is surrounded by lighter squares which make it look darker, and in the other case it’s surrounded by darker squares, making it look lighter. The shadow going across the squares messes with your perception as well, amplifying the effect.