Content - Hermann Grid
The pattern consists only of black squares with white surrounds. However, if you fix your eye on a particular intersection, the surrounding white spaces appear darker. another example
The fuzzy grey squares you see on the Hermann Grid are caused by lateral inhibition - the response of each area inside your eye is reduced by the light falling on neighboring areas around it. This minimizes your eye's response to constant brightness, and exaggerates the light differences at edges.
This effect appears only in your peripheral vision. It depends upon the natural motion of your eye, and the scale of the pattern.