As an advertiser, there are often times when you want to site target a particular site running AdSense, but your particular product or service is really only relevant to a specific portion of the site. Or you might only want your ads running in a specific ad unit, such as the one running above the fold front and center. Google has released a new custom channel targeting feature that will allow advertisers to do that.
Publishers will need to turn the feature on for each relevant custom channel in their account, or set up brand new custom channels, breaking down the site or ad units in ways they feel advertisers will want to target them. Then advertisers can access the channel information for specific publishers in order to pick and chose what to target on the site.
Since the feature has just been launched across all publisher sites (although it was available previously as a beta test) it will likely take some time before many publishers adopt this new feature. Publishers need to specifically turn on the feature for each channel they would like to use before they will be available for advertisers to target.
There is more detail on JenSense but Google has not released much information on it yet for either advertisers or publishers.