Tier 0 is used to describe the best deck. When there is a Tier 0 deck in Modern or Standard, it means that the only way to beat it is to either play that deck, or a deck that specifically counters it.
While the people who run the TappedOut list are getting better, they do have strong biases and an aura of elitism. Most of the placements make sense, but other times they are super adamant on the positions of certain commanders, despite what the numbers show. They only recently brought Godo up a tier, and that deck has been around for a year now (since Dominaria). I still have reservations about the placements of
Yisan, the Wanderer Bard and
Taigam, Ojutai Master, but at this point it's minutae.