The Very Special Episode is when a show deviates strongly from its normal premise to highlight a social issue, cause, or other thing. So the sitcom about nominally wealthy kids getting into wacky hijinx suddenly focuses on the poor kid they've been making fun of, and the main crew Learns A Valuable Lesson about not making fun of the poor kid because he's poor (one they will likely never mention again, because the poor kid still "deserves" to be made fun of), then you have a Very Special Episode.

Cartoon All-Stars is my go-to on this, because it was all about the kid and his broher being Saved From Drugs, thanks to the combined power of many Saturday Morning Cartoon characters acting in their sudden anti-drug stance. And the kid telling his mother ab9ut his brother's drug paraphenalia.