Watchdog journalism is the idea that the media is free from any government restriction. This makes it possible to both "listen" and "watch" what public officials say and do without backlash consequences. It's important for the media to point out "spin," answers given by bureaucracy that are shady and are in fact not an answer to the question posed.
Watchdog journalism is also referred to a form of activist journalism. It's the same idea, hold public officials accountable. Just like the media is the fourth branch of government, capable of operating as an additional balance of power.
"In recent history, a notable example of watchdog journalism was the exposure of Dan Rather's investigative segment which cast George W. Bush's military record in an unfavorable light. The segment was based on the Killian documents, which blogger journalists exposed as being insufficiently verifiable as authentic." (Source)
On a personal note, I'm a reporter who has frequently attempted to hold public officials accountable. I did a series of stories on a chip seal problem in Idaho, where the Idaho Transportation Department kept using a chip seal despite it frequently failing.