Comment on Google AdSense Ad Channels by SEO Dave.
What this means is the main content of a page is enclosed in the following code supplied by Google AdSense:
!– google_ad_section_start —
Your important content that you want Google AdSense to consider important when choosing keywords.
!– google_ad_section_end —
What this is meant to do is Google AdSense will look between this code and try to use the content within to target your AdSense ads.
The theory is the content you write is going to be more keyword targeted page by page than using that content and all the template type content, links on the menu etc…. So by enclosing it in this code Google AdSense should target your content better which should result in better ads which should result in a higher CTR.