Most often Google AdSense place ads on your site that are relevant to your content but sometimes Google gets it wrong. It can also be times where you don’t want Google to put any relevance to a certain text on your site when they crawl it.
An example would be if you write a blog post about that you don’t like poker sites. Then it would be unfortunately if Google showed ads for poker sites on your blog.
With Relevant AdSense Ads there is now an easy way of helping Google distinguish which content that you think is relevant and which content should be ignored.