The AdSense Heatmap is a very simple tool for understanding how to create high-performance ad layouts.
The heat map attempts to show areas of an average layout where site visitors focus their attention. The darker areas get more attention, the lighter areas get less attention. As a result, ads in the darker areas deliver higher CTR and ads in the lighter areas deliver lower CTR.
Any one heatmap is a rough guide, of course, as every layout is different. But, as a first tool, understanding this can really help you improve your CTR and CPM very quickly.