If you're an Adsense publisher you may find that the ads sometimes aren't relevant to the main content of your page. This can happen, for example, when one part of the page is interpreted by the Adsense bot as being more important than it really is. It's happened to me a few times - I remember including a reference to a particular product as an aside in an article. The Adsense ads were all about that product instead of the article topic.
You can get around this problem using section targeting. Add the appropriate tags to the page content to tell Google which text should be used (or ignored) when targeting ads.
Of course you also need to make sure the page does contain the sort of text that will attract the advertisers you want, but I assume you knew that already :).