What this means is that Google is encouraging you to follow good website development practices. If you fill your site with pages that have good content, use relevant keywords, and clearly telegraph the topic of your site, then you'll tend to succeed on a couple of levels.
First is you'll tend to draw visitors from the search engines, because your pages will be understandable by the search engines, and they'll be able to match your pages with the searches entered in the search engines. The second is that Google will readily match the advertising to your page.
A rule of thumb might be that if the advertising Google shows for your page isn't relevant, then you've done something wrong with the content of that page. It isn't quite so simple as that, as we'll discuss in a few minutes.
The last point is the economics model points to a way to maximize your Google revenue. Simply write about hot items that carry a high CPC value. The advertising showing up on your page would tend towards high CPC advertising, and you'll tend to earn more that way.