An Anchor Ad is a type of overlay ad which stays at the bottom of a webpage regardless of scrolling. They often have a way to close them or expand them into larger ad units
Anchor ads can both run on desktop and mobile web traffic , on desktop it will be 728×90 ad, while on mobile it will be either a 320×50 or 320×100 ad.
As its an overlay ad, it will be always be above content and possible other ads, so publishers must be cautious with implementation.
Being an overlay, anchor ads will have high viewability and thus can generate good revenue.
Nonetheless it is important to not overload the page which could harm user experience , thus lower revenue on the long run.
Below is an example of anchor ads on desktop and mobile.
Desktop anchor ad