The phrase "jump the shark," referring to the point at which a television show went into decline, came from a season five episode of Happy Days in which the Fonz jumped over some kind of marine animal on water skis. After four years, this nostalgic look at the 1950s was one of the most popular shows on television. After Fonzie made his jump, alas, things were never quite the same.
Ron Howard still had top billing during season three of Happy Days (which aired on ABC in 1975-76) but Henry Winkler now came second in the opening credits, and had clearly become the real star. Arthur Fonzarelli, a leather-jacketed tough guy so cool he could start the juke box simply by hitting it, was introduced as a supporting character in season one, but he was popular enough to save the low-rated show from cancellation.
Winkler struck just the right balance between making the Fonz menacing but likable, and played the role with absolutely perfect comic timing. It's easy to see why he became so popular, but the producers were really starting to overdo it by this point. He'd moved into an apartment over the Cunninghams' garage, and had basically replaced Chuck Cunningham (Remember him? Not many do...) as Richie's big brother.
Whenever the writers couldn't think of any other way to end an episode, they simply got Winkler to snap his fingers or something, and everything came out alright. In the episode where Richie and his pals try to meet girls by staging a fake beauty contest, for example, the guys are about to get strung up when the contestants find out the grand prize - a trip to Hollywood - doesn't actually exist. Fortunately, the Fonz shows up and appeases the winner with an even better prize: a month of dates with the Fonz, of course!