AdSense is a "contextual advertising" program and when it is working at its best it is highly effective because it serves up ads that are highly relevant to the content on your site - this results (ideally) in a higher CTR (click through rate).
The problem is that AdSense doesn't always get it right when assessing what your content is about and it will from time to time serve up irrelevant ads.
AdSense acknowledge the problem of irrelevant ads and have come up with a way of helping their "bots" determine what parts of your site you want the ads to relate to (section targeting).
Section targeting allows you to put code around the most important parts of your page (ie the bits that you want the ads to relate to) and also allows you to put code around parts of your page that you don't want AdSense to consider when serving up ads.
