An interstitial is an ad that takes up the entire screen of a website and forces users to see the advertising before they can see the content on the page.
Sometimes interstitials appear before a page is loaded.
Interstitials for Google
In early 2017, Google introduced a change to mobile search results, which would be bad news for the web that uses interstitials. Thus, it ensured that such pages that used interstitials that block user content, would lose visibility in mobile search results. An announcement, which of course, does not affect all interstitials.
The interstitials that are allowed
Legal interstitials for cookies and age verification, etc.
Logins for content that is not publicly indexable
Discrete banners that can be easily discarded and don’t take up too much space, such as app install banners in Safari and Chrome.