Google has a lot of ways to enforce relevance across their platforms. In the Adwords world, one of the many tools they use is the quality score. The quality score applies to keywords in the Google Search Network. Basically, it’s their way of determining if the keyword is relevant to the ad content and the landing page. This helps them prevent people from bidding on totally unrelated keywords and promoting their ads, thus keeping the ad network relevant to the searchers query.
All in all, this is a good tool. One great way to take advantage of the Google quality score is to sort adgroups in REALLY tight themes and make sure you have a themed landing page to accompany each individual landing page. This will make you keywords, ad copy and landing page super related and relevant to each other. When you do this, it helps increase your chances of a high quality score. With a high quality score you can bid much less for your placements. Any successful long-term campaign thinks long and hard about utilizing the Google’s quality score and you should do the same.