From January 10th 2017 Google are adding some new filters to their mobile search engine algorithm to not rank pages as high if they have pop-up ads on their pages. Usually these come in the form of ads or newsletter sign up forms. These can be particularly distracting for mobile users as there is nothing else to see on the page but the ad, often with a tiny dismiss button that is easily mis-clicked/tapped.
Pages that show intrusive interstitials provide a poorer experience to users than other pages where content is immediately accessible. This can be problematic on mobile devices where screens are often smaller. To improve the mobile search experience, after January 10, 2017, pages where content is not easily accessible to a user on the transition from the mobile search results may not rank as highly.
As ever this change is about making the web more user friendly experience and reduce the annoyances that have developed over time. It is important to bear in mind this is only a change to the mobile search results page not the desktop one, but as more and more people are using their mobiles to browse the web, it’s better to be safe than sorry.