“Non-viewable impression distribution” represents the percentage of your total ad impressions that were considered non-viewable. Total impressions include both measurable and non-measurable impressions. This can help you understand how many of your ad’s impressions couldn’t be viewed.
It shows the number of times your ad was considered non-viewable. An ad is non-viewable when it doesn’t have at least 50% of its area visible for at least 1 second for Display Network ads, or 2 seconds for video ads.