The AdSense Tracker tracks the referring URL for every AdSense click. The referring URL or "referer" is the address of the page the person was on before arriving at the page where they clicked your ad:

This information can be very useful for planning purposes. You may find that a large number of ad clickers are arriving at your site from a small handful of sites or search engines. If you can increase the number of links or visitors from those places - or create similar pages that attract similar traffic - you may be able to increase your overall clicks and revenue further.