Interstials are full-screen ads that cover the entire page of a mobile site. They block the entire user interface and can be very intrusive even though they can also be well paid at times. 

At Marfeel we strongly discourage their use because of their intrusive nature that detracts the user experience. In addition, Google also announced that starting in 2017, mobile sites that use them will have their ranking impacted because they are not requested interactions or obligatory actions like the disclosure of a cookie policy or legal disclaimer.