"Jump The Shark" is the moment something (usually a TV show) begins to suck. On Happy Days, in the later years, there was an episode where Fonzie actually jumped a shark on water skis. This is sort of held up as a perfect example of a show having run out of ideas.
Other people suggest that the true "Jump The Shark" moment for Happy Days was when Ted McGinnley joined. (Strangely, Ted McGinnley is a Jump The Shark candidate for several other shows, including Married With Children and The Love Boat).
Anyway, the Jump The Shark idea has taken off. There's a pretty funny website about it all...
http://www.jumptheshark.com/