-Many bookmakers offer a welcome bonus, but in order to be able to withdraw those bonuses, first you have to complete the bonuses rollover requirements. Which means, that you need to bet the value of the bonus a few times, before being able to make a withdrawal.
The objective is, to be able to bet the bonus as many times needed, without loosing it.
And how can you do that?
Imagine a football match between France and England. At the bookmaker you have your bonus, you use that bonus to place a bet on the victory of France. And at Betfair, you bet against France, by placing a lay bet on France. You need to use a special calculator, to know what is the right amount of money to place on your lay bet at Betfair.
This way, all possible match outcomes are covered by your bets. Therefore, you will win at one bookmaker, but obviously, loose at the other. You can see it as a transfer of money from one bookmaker to the other.
In case you win at Betfair, congratulations. Now all your money is at your Betfair account, and you can withdraw it.
If you win at the other bookmaker, just repeat the process. Until you win at Betfair, or until you complete the bonus rollover conditions.
And then, you will be able to withdraw all your money.
-And this is what your objective is, to be able to bet your bonus as many times needed, without loosing it.