This optical illusion illustrates that the same luminance can be perceived with different brightness depending on the luminance of the surrounding objects. The grey rectangles shown all have the same luminance. The rectangles surrounded by black stripes appear brighter than those between the white stripes. It is important to note that this effect is the opposite of what is expected based on the simple physical explanation of simultaneous contrast. With simultaneous contrast, the rectangles whose long sides border the black fields should appear lighter. 

You can see that the grey rectangles are the same colour if you look at the picture with a blurred view, i.e. "through the picture", so to speak. The grey fields move until they are directly next to each other.