Google has announced that they will be changing the way they rank mobile sites based on how they use interstitials (pop-ups that block content).
This appears to be focused on making the mobile web user experience better for those that use Google search.
Mobile websites using interstitials that block access to the content the users are expecting will likely see a drop in their rankings after Google flips the switch on this expanded feature on January 17th, 2016.
There are certain types of interstitials that will not negatively affect the rankings. They are sensible implementations such as legal or terms-of-use notifications, login screens, or banners that don’t cover a large screen area and are easily dismissed by the user.
These changes make sense from a web development standpoint, and appear to be working in favor of the user. You will want to make sure that your website doesn’t break any of these new guidelines moving into the future if you rely on interstitials to present content or ads to users.