When you are given a Welcome Bonus, you will find that they have playthrough requirements attached to them. What this means is that you will need to roll over the bonus money a certain number of times before you will be eligible to withdraw any winnings that you have made from the bonus. So, to give you an example, if the playthrough requirements are 5x and you have received £10 as a bonus, you will need to wager £50 before a withdrawal is possible.