This sounds like pretty standard retargeting. If I have the retargeting code on my site it drops a cookie on the user which, when they visit other related sites ensures that my ad shows to those users. My ad might show anyway but it sounds as though this ensures that it shows for those users who have been on my site previously. I use a similar service for display ads. If a user visits my site, then visits another site showing ads from that network, they see my display ad. My ad doesn't show for all users on that other site, only ones who have been on my site previously.