In a tounament what is the tiebreak?

To answer your question:

The tie break is only used if two or more players share the number 1 spot on Score. If two or more has the same Score, then Tie Break will decide who advances to the next round. If two or more people share's the same Score and Tie Break, they will both (all three) advance to the next round.

You'r Tie Break is calculated by you'r opponents Score, if you beat someone who has a score of 7, you will get 7 Tie Break points.

I suspect it's also used to seed the match ups for subsequent rounds even if the tied players don't share the number one spot.  I can even mean the difference between advancing or being eliminated in some cases.

Seeding is based on rating of the advancing players. The tie break is not involved. (The description in help is inaccurate. Seeding starts from the lowest rated player being placed in the last group and working upwards.)

Players tied for second or lower advance if the tournament setting "# Advance" indicates more than one.

In that case, it really does only ever matter if they players are tied for the top spot.

There are more tie breaks like the progress tie break that adds up all your running scores in the tournament.

Eg.  

   Round 1  Won     Total 1

  Round 2 Lost  Total 1

round 3 Won Total 2

Round 4 Drew Total 2.5

Round 5 won Total 3.5

1+1+2+2.5+3.5=10

So your tiebreak for progress is 10.