In the wonderful world of TV, an interstitial is something that shows during commercial breaks that isn’t a commercial or something required, like a station ID an Emergency Broadcast System Test (not that we have those to worry about anymore, thank goodness). It might be a brief announcement of what network you’re watching, like this:
Or a brief news update:
Or the community bulletin board:
The ABC television network in the US used to have an interstitial that ran during the afternoon called "FYI," hosted by Hal Linden, star of that network’s Barney Miller.
Little shows like these are pretty common on kids’ TV. The Dutch, for example, had an animated character named Loeki de Leeuw (Loeki the Lion) who showed up during short breaks in TV shows. I think they tried him here in the US, and it didn’t go over very well.
CBS had short news stories that they fit in during the Saturday morning cartoons in the ’70’s and ’80’s. This is a news report about ice skating champion Diane de Leeuw, no relation to Loeki.
The Kaiser Broadcasting stations (of which WFLD in Chicago was one) had a number of little shows called "Snipets."
There are plenty more examples, and maybe you have some examples of your own, so feel free to include those in the comments.