As part of their continued effort to make mobile internet usage more user-friendly, Google changed their search algorithm to penalize pages that have “instrusive interstitials,” which refers to popups and popovers that cover the majority of the visible area. “They can frustrate users because they are unable to easily access the content that they were expecting when they tapped on the search result.”
Beginning 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.”
Google has included these example graphics on their official blog post.
If you use a pop-up or pop-over on your site, which are commonly used to encourage list subscription, you should evaluate it on mobile devices to see if it covers most of the visible area. If so, it’s probably in your best interest to look into using a different pop up plugin, or one that can be better optimize for mobile usage.