You can roughly divide bonuses you get at the casino into two categories. The first category is the so-called cashable bonus or non-sticky bonus. That is a bonus that you can have paid out (after possibly meeting wagering requirements).
The second category concerns the sticky bonus. A sticky bonus (as the name says) sticks to the casino and is therefore never withdrawable (even after having met all wagering requirements).
The sticky bonus is usually a no deposit bonus. To claim a no deposit bonus you do not have to put money in your account yourself and you can play carefree with your bonus money. It can also occur as a deposit bonus
Example of a sticky (deposit) bonus
Suppose you have deposited €100, where you received a sticky bonus of another €100 from the online casino. You then have €200 in your account. Next, you will play for a long time so that you have met the wagering requirements and you have managed to collect a total of €300. That is, therefore, €100 deposit, €100 bonus, and €100 profit. If you now want to have your money paid out, you can only withdraw a maximum of €200. The sticky bonus of €100 will stick to the casino.