To 'jump the shark' is a verb applied to a television series, typically. It indicates the point in the series' on-air run when, in the speaker's estimation, something critical happened that caused the show to begin the long (or not so long) but inexorable slide downhill to the status of junk. A show has jumped the shark if it has reached one of these points. Examples are said to include:

There are even specific categories of how to jump the shark. For instance, the main characters can hook up, as in #1 above. Or a show can resume after a season break - but with a different actor playing a continuing character. Or perhaps, like in Dallas, an entire season can be declared to have not happened!

Thanks to the world wide web, there is a whole website where you can discuss, research and sound off on when shows jumped the shark. I am told, however, that TV Guide took over the site and performed all manner of unspeakable changes on it, so I can't recommend it.