According to the official statement, interstitials affected by Google's crackdown include the following:

• "Showing a popup that covers the main content, either immediately after the user navigates to a page from the search results, or while they are looking through the page.
• Displaying a standalone interstitial that the user has to dismiss before accessing the main content.
• Using a layout where the above-the-fold portion of the page appears similar to a standalone interstitial, but the original content has been inlined underneath the fold."