What is AdSense Section Targeting?

Your friend is definitely on the ball with this one! I haven’t even had a chance to talk about it on my Google book blog (at findability.info) yet, though that’ll be remedied in just a few minutes! 🙂
More seriously, Google’s allowing AdSense participants to help their system identify exactly where the content, the real key material on a given page, is located. The value? This better targets ads, which helps visitors since they see more relevant ads, helps the advertiser because it improves the targeting of their ads, and that, of course, improves things for us participants too, because we should see better click thru rates and higher revenue.
There are two different types of tags you can use, though I surmise that the basic pair will be fine for 99% of users.
Just before the beginning of your main content, the material that you want to have be used as the basis for targeting, add:

then at the end of the passage, add:

Easily done, and doubly so if you’re using a template-based system like a weblog, since you can just tweak your templates and rebuild your site, whether it’s a dozen pages or a thousand.
If you have sections that you absolutely want Google to ignore, then change the ‘start’ tag to:

That’s it. Now, go implement it!
Oh, and remember that you won’t see a change in ad targeting until the Googlebot next visits your site…