Google recently introduced Active View metrics in Adsense accounts, to track the viewability of ads served by AdSense: that is, the percentage of ads that are considered viewable out of the total number of ads measured.
The Google’s ad viewability measurement solution uses the online ad industry standard: a display ad is counted as viewable when at least 50% of the ad is within the viewable space on the user’s screen for one second or more.
So an ad that is (for instance) placed below the first screen on a given page won't be counted as viewable if the viewer never scrolls down. This is why viewability is a good way of determining whether your AdSense ads are actually being seen by your website visitors or simply loading without a chance of catching the viewer's eye.
Benefits for Advertisers
Without viewability metrics, advertisers could be paying for ads that loaded on a page but were never seen. If you're an advertiser paying per click you might not care, but if you're paying by the impression it could be a much bigger deal.
Active View metrics help because they show advertisers what percentage of their ads are actually being seen by users and which publishers provide the highest level of viewability. This helps advertisers make smarter decisions about where they should buy ad impressions. That makes display ads both more measurable and more effective.
Benefits for Bloggers ( Publishers)
Viewability metrics also help publishers understand the true performance of the AdSense ads they display. Whether the ad is being paid for by click or by impression, ads that aren't seen by the user can’t have an impact. In the long run, they won’t generate reliable ad revenue.
If you're a blogger and notice that the Active View percentage for certain ad units is lower than others, that ad position is probably underperforming and ought to be optimized. It's a good idea to start paying attention to these new numbers and make adjustments to your ad sizes, ad units, and ad positions to improve viewability and potentially achieve better results.
Paying close attention to your Active View numbers will help you bring your visitors the targeted ads you want them to see. In the end, that should translate to improvement in the performance of these ads and a potential increase in your AdSense earnings.
Source : Adsense Help Center