Bonuses for casino offers can often have wagering attached to them, or you need to complete wagering to get a bonus or both.
Wagering isn't how much money we have to lose, it's simply how much we need to play through.
For example, 100 x £1 slot spins would be £100 wagering.
Sometimes wagering is given as a monetary amount (eg. wager £100), sometimes it is given as a multiplier (eg. the £5 bonus must be wagered ten times).
In this scenario, we multiply the bonus amount by the wagering requirement to get the total amount that needs playing through. Here it is £5 x 10 = £50.
At times you will be required to wager just the bonus, other times there will be wagering requirements on both your deposit and the bonus given to you by the casino.