Viewable cost-per-thousand impressions (vCPM):
- applies to all ads that appear in Google Search and on the Display Network
- lets you pay only for the impressions that become viewable
- lets you bid based on AdWords’ projected views of your ad
- counts an ad as “viewable” when 75% of it is visible on the screen for 5 seconds or more
Explanation: If you’d like to pay only for ad impressions measured as viewable, you can with viewable cost-per-thousand impressions (viewable CPM). An ad is counted as “viewable” when 50 percent of your ad shows on screen for one second or longer for display ads and two seconds or longer for video ads. You can select Viewable CPM as a bid strategy when you choose CPM bidding for your “Display Network only” campaign.
Read more here: https://support.google.com/adwords/answer/3499086?hl=en