An app interstitial is an advertisement for an app that covers all the content on a page when you visit the website from a search result on a mobile device. These app interstitials may also stop a user seeing the content until they download and install the app.
Customers who have clicked a link to view content should not be blocked from viewing the content or forced to download an app to do so. Blocking content or forcing the download of an app is a poor customer experience. Further google has indicated that websites that undertake this practice will get their organic results downgraded.
There are alternatives to using an app interstitial such as a smart app banner in Safari or a Native App Banner for Chrome. You can also use an HTML banner that is typical for the device and that banner can then connect to the app store.