If you are looking for a show specifically using the notation (often used for mid-season recaps or specials), it likely refers to one of the following:

This episode, which originally aired on February 26, 2012, is best known for its heartfelt and quirky tribute to the Swedish car manufacturer , following its bankruptcy.

Episode 18.5, titled "All of This," is a recap episode that summarizes the events of the series up to that point. Community consensus on Reddit generally suggests it is skippable if you are binge-watching.