It's a term coined by Maajid Nawaz to describe a phenomenon with the contemporary left, whereby people resort to ends-justify-the-means style tactics to smear opponents and avoid honest intellectual discussions about perceived politically-incorrect issues. It's characterised by de-platforming opponents, misquoting them, using straw-man arguments, and other anti-intellectual tactics, implicitly asserting one's moral superiority instead of grappling with the facts as they can be understood in honest journalism or debate.
It seems to be a form of intellectual tribalism. It's a problem because it inhibits genuine discussion of the world as it is, undermines trust in journalism, and encourages poor styles of thinking and widespread adoption of bad ideas. I think one of the most well-known and biggest offenders is Glenn Greenwald.