Google does a very good job of displaying relevant Adsense ads that matches your blog or websites content. But there are exceptions to this and if your blog is getting ads that doesn't match your content then you can make them more targeted using section targeting. Section targeting is using some specific tags to instruct Google bot not to consider some sections when displaying ads or instructing Google bot to specially consider this section when displaying ads.Although section targeting is a powerful method the title of your blog or blog post is the most important thing to get relevant Adsense ads so make sure you add some relevant key words to the title first.

If you want to highlight a certain section you can use the following tags.

<!-- google_ad_section_start -->
content to highlight
<!-- google_ad_section_end -->

If you want the Google bot to ignore certain section you can add (weight=ignore) to the tag

<!-- google_ad_section_start(weight=ignore) -->
content to ignore
<!-- google_ad_section_end -->

Keep in mind that this method should be used to highlight important sections of your blog and it shouldn't be used to get ads for high paying key words. Also you need to add a considerable amount of content in the targeted section for this to work properly. Insufficient content may lead to you getting irrelevant ads. This is what Adsense support has to say about section targeting.

In order to properly implement this feature, you'll need to include a significant amount of content within the section targeting tags. Including insufficient content may result in less relevant ads or PSAs. In addition, please keep in mind that this feature should only be used to emphasize significant sections of your site's relevant content. It is against our program policies to manipulate the ad targeting to result in ads that are not relevant to the content of your pages.