It's based on the number of wins you have in that tournament.
For every game you win, your tiebreak goes up by the SCORE of the player you beat. For every game you draw, your tiebreak goes up by HALF the SCORE of the player you draw against.
http://www.chess.com/tournaments/help.html#tiebreak
(There are also tournament points which are completely different).
During a tournament i have nine points of tie break. What do they mean? Thanks