In Bookmaker mode, you get to be the "bookie" and set your own odds. Setting odds enables you to potentially get better odds than what's currently being offered, but does not guarantee that your bet will get matched. Be sure to check before the game starts to see if it got filled.
For example, if the best offer available on the “Over 222 Points” for an NBA game was 1.90/-110, and you wanted 2.00/+100, you could go to bookmaker mode and create a bet offer with odds of 2.00/+100.
In this scenario, your offer would go to the top of the order book. This means the next time someone goes onto bettor mode to bet, they would match your order, and you would both get 2.00/+100 on the game.