Interstitials are usually ads that overlay the web page, often during a natural transition point, such as loading a page or scrolling. What makes them interstitials is how they cover most of the page, with other types coming in the form of banners, popups and dialogues.

An interstitial can become a problem for SEO when it starts to impact the useability of the site, something that Google’s Core Web Vitals are trying to measure.