Did you notice that there are two lenses listed at top? If the arrow is pointing left.. the image you see is from the the lens listed on the left... and vice versa. Make sense? Hypothetically, if you look at an image from each lens, at the same ISO, FL and Aperture... it should give you some idea of the relative sharpness. But since the images seldom seem to have been taken under the same identical, controlled conditions... the comparison is probably not very accurate.
There is no "reading" to it. The "diagram" is merely a photographic target that is intended to show sharpness and other qualities of a particular lens. The idea is that one can take pictures of the graphics and compare the resolving power of the lenses.