The power of Adsense is that the ads are targeted at the content of your website, and thus, normally, at the interest of the viewer.
However, depending on your website setup, this can give some problems sometimes. For instance, if you have a photoblog, there isn’t much text for Adsense to see what topic your blog is about. Same goes for other sites that do not have alot of text, or where the real content can be overwhelmed by other content, such as comments, navigation, other ads etcetera.
Now, there is a solution to this problem, since you can suggest to Google Adsense which parts of your website you would like to have your ads targeted on. This is called section targeting.
The html code to put an emphasis on a certain part of your website is this:
<!– google_ad_section_start –>
Your content you want targeted goes here!
<!– google_ad_section_end –>
In fact, the code takes a parameter, weight, which you can set to “ignore” (standard it is set not to ignore, but to emphasize):
<!– google_ad_section_start(weight=ignore) –>
You can use this around the html code for your menu, or other advertisers.
The section targeting code can be used in a single page as much as you want. However, it must match the content of your site. If you use it to drive other topic (probably higher paying) ads to your site by targeting false keywords, you are working against the Google policy, and you could get your account closed.