If you do not know what "Jump the Shark" means, it pretty much means the point that the strayed from the formula that initially made it a hit.

For example...

Roseanne - I think it jumped the shark around the time Becky married Mark and left, and David moved in. From there, the show started focusing too much on non immediate family situations. After that it seem like everyone got pregnant, was gay, came from a dysfunctional background, etc., etc. It lost it's down-to-earth appeal about a middle America family that was hard working and something everyone could relate to.