Viewable impressions allow you to:
- pay for those ads that are actually viewed
- pay for ads that are viewed and also converted
- pay for video ads that are viewed within an hour
- pay for ads when they are displayed in a viewable position
With vCPM bidding, you bid for your ad based on how often it appears in a viewable position on the Google Display Network.
Read more here: https://support.google.com/adwords/answer/2630842?hl=en