The unseen impression filter is an AdWords feature that allows advertisers to prevent their ad from appearing in ad units on the Google Display Network that are not seen or clicked on by users, despite ample opportunity. This includes ad units on AdSense publisher websites.
The intent of the unseen impression filter is to make advertising on the Google Display Network more appealing to brand-focused advertisers who want to have their ads seen by as many users as possible across our network. By attracting more new advertisers and increasing the spend of existing advertisers, this should improve revenue for publishers.
To maximise your AdSense earnings, we recommend that you place your ads in highly visible areas. Learn more about finding the optimal location for your ads to ensure that they're seen by users.