What does tie breaks mean and what do they do?

Tie breaks are generally used to determine a winner of some event where a technique is used to separate the players who otherwise have equal scores. You usually see this in a tournament where maybe there are only a few rounds of play like 4 or 5 rounds and 2 players have equal scores.

 A variety of tie break systems can be used and most tournaments will inform you up front which system will be used. An example of such a system would be a common opponent. Suppose you and another player played a 5 round tournament and you both had the same number of points. Supposed you did not play each other during the tournament but you both played the same person.

If you won your game against that person and the person whom you are tied with points wise got a draw against that very same person, you would get ahead as a result of the tie break system if the one that I mentioned was used. 

kk thx i get the general idea but in my tourney someone has a tie break of 4 and i have 2 but we have played the same amount of games and won the same amount

They could have used a system where they removed the the highest and lowest points opponents from each of you and then counted the points of your other opponents to see who faced the tougher competition.

You should read http://www.chess.com/tournaments/help.html#tiebreak for more information on the tie break system used for tournaments on Chess.com