Jumping the Shark is colloquially used when writers and producers run out of story ideas and instead reach for cheap methods to create an impact. It was first attributed to Happy Days when in a latter episode The Fonze literally jumped a shark. It was seen as a sign of a decline of a much loved show. I could also be attributed to the last series of this show.
I can see where the writers were going with this series, they just whiffed in the implementation. It was a total volte-face in the style of show this was. From a sober drama to something akin to an action show. In the process they broke the characters' personalities and motivation. Much like Game Of Thrones season 8 did much later on.
Don't fall for the stinger at the end of series 2 and avoid watching this series. Or, if you do lower your expectations so they are scraping the floor for this flawed end to the series.