It's a game played by two players where one of the goes "You call that a nine? This is a nine." and presents his nine or dubs. Then the other player responds with "That's not a nine, that's an X" if the first player mistakenly presented a single that wasn't a nine. Next the second player proceeds to present his own nine or dubs.
The winner is the one with more nines or better nines like dub nines or trip nines. Normal dubs and trips don't count for points, but you don't lose with them.