Section targeting is a method of telling the adsense engine which parts of a webpage to focus on, to select the keywords used for matching the ads to display for the content. Section targeting is considered to be effective in showing relevant ads for a website, although the results vary across websites.
The process to implement it is quite simple. Any part of the content that adsense should use to select keywords should be wrapped in the following html tags.
<!-- google_ad_section_start --> ... content goes here ... <!-- google_ad_section_end -->
For wordpress the most important part a page is the post content. So it is a good idea to wrap the post contents in those tags. A plugin is not necessary to do this if you are willing to edit the theme files right away. Just open the single.php (the theme file for a single post) and look for the line
<?php the_content(); ?>
edit it like this
<!-- google_ad_section_start --> <?php the_content(); ?> <!-- google_ad_section_end -->
If you are also displaying tags at the end of a post then wrap them up too. Also wrap the title like this
<!-- google_ad_section_start --> <h1><?php the_title(); ?></h1> <!-- google_ad_section_end -->
Section targeting also allows to mark certain contents to be ignored. To do this just wrap the particular content in the following tags
<!-- google_ad_section_start(weight=ignore) --> ... content to be ignored goes here ... <!-- google_ad_section_end -->
So what can be ignored on a wordpress blog. Well, if you have having lots of links and other text in your sidebar or widgets they can be marked to be ignored since they do not always relate to the main topic/content of the current post. It might be a good idea to exclude the navigation menus, social links etc too since are always constant on all pages.
Having done all that, the effects may not be instant. Observe the ads for the next few days or more than a week and see if they improved. If section targeting really improves the keyword selection to match ads, then it might improve the CTR as well.