Google adsense is the onsite advertising that Google does off-search (i.e. other peoples websites). It uses a special system to calculate what the page is about. If you signup for an account or use your existing account and put the adsense code on your site you get a little seo help….
How? Well it will tell you what google thinks your page is about, once your idea of what the site is about and googles is the same then take the ad off. (Unless you want to make some money on a non-commercial site).