Interstitial ads are interactive, full-screen ads that cover the interface of their host app or site. These ads appear between content, so they place at natural transition points or breaks, such as in between activities or game levels. Their full-coverage is what differentiates them from other ad types, like pop-up, native, and banner ads.

I still use exoclick in my wordpress website. Even thought the rate is low, you can fill up some empty spaces with the amount of exoclicks ads and options.