Mediavine Adhesion Ads are the ads that stick to the bottom of the browser as you scroll through the content. Adhesion ads are enabled as a default setting on desktop, mobile, and tablet, have terrific viewability, and because of that also tend to have very high CPM's. 

In your dashboard ⚙️ Settings > Ad Settings you will see this option:

The close button offers the user the option to close out of the adhesion ad which is a better user experience. The close button is DISABLED by default. 

The recommended setting (Adhesion Close Button OFF) increases adhesion revenue by an average of 7%, and up to 10% in some cases. It's a quick win, especially since MOST sites run adhesion units and readers are accustomed to them.

The Adhesion Close Button Setting applies to ALL of your adhesion units on desktop, tablet and mobile.

If you are looking for a slightly improved user experience, you can ENABLE the Adhesion Close Button so that users have the option to close it out.

It is important to carefully monitor adhesion earnings, adhesion impressions, and time on site after implementing this change.