At an online casino, you will have two different kinds of balances. Deposit cash is the money from your pocket – it usually comes with a 1x wagering requirement, whether you claim a bonus or not. If you deposit $/€/£100, you will have to make wagers worth at least that amount before withdrawing. This is to prevent criminals from using online casinos as a front for money laundering.
Bonus cash balance is what you have when you claim a match deposit bonus. This balance will only be used once you have used all the deposit cash in your account.
This is what happens when you claim a bonus and plays:
You deposit $/€/£100 and get a $/€/£200 match bonus.
You play some slots – first, the deposit cash will be used for placing the bets.
Once that $/€/£100 is finished, the casino will start deducting from your $/€/£200 bonus balance.
This is the point where the wagering rule kick in – each bet you make with bonus funds will contribute to your wagering target. In general, bets made with deposit cash do not contribute to the wagering requirement. There are exceptions, but those casinos are quite rare.