Section targeting is used to help make your Google Adsense ads more relevant to your content, and in turn improve your clickthrough rate (CTR). To target a section of your site, simply surround it with these HTML comments:

<!-- google_ad_section_start -->

<!-- google_ad_section_end -->
This is important to do, because other content on your site, such as in the header, side bar, or footer, may not be relevant at all to the ads you want displayed for a specific page or blog post.

By targeting just the relevant content of a specific page, you can make sure the Adsense ads that are displayed are as relevant as they can be, which will improve your "clicks" and hopefully annoy your visitors less.