When a solid object is in front of the sun, the parts which are not directly in front of the sun are accentuated by the lack of light and appear as solid black objects.
If the moon were a solid object, then when it passes in front of the sun in a solar eclipse, we should see the part of the moon that is not directly in front of the sun as a solid black object. That is what this picture is trying to illustrate. I understand that it would not actually appear as a completely solid black circle (that the section in front of the sun would appear quite light), but otherwise it shows the difference between what we should see if the moon were a solid rock versus what we actually see during a solar eclipse.
