When you join a new casino you will be presented with some sort of welcome package, this can comprise of additional money on top of your first or more deposits, as well as extra spins on specified slots.
When you read the terms & conditions of the bonus you are accepting, you will see that it will state what the wagering requirements are. For example let's take the Casilando welcome bonus, they give a 200% match on your 1st deposit, so let's say I have just deposited €50 with them for the 1st time.
€50 + 200% match = €150 to play with.
Wagering requirements are x35 deposit + the bonus amount.
So under the terms & conditions of this bonus, I need to play through €5,250 before I am eligible to withdraw any money from my balance. The terms on these bonuses vary across online casinos and different types of bonuses are also offered. You can read more about them here in online casino bonuses explained.
Years ago the wagering requirements on these bonuses were a lot less restrictive, it's absolutely vital that you read through the terms & conditions of any bonus that you accept. There will be a maximum stake that you are allowed to bet during your wagering, this is usually around €5, there will also be restrictions on certain games that you can play, and some games may only count as a certain percentage toward your wagering.
These are always listed and explained in the T&C's of each individual casino that you will play at. When you make a withdrawal your gameplay is always checked, so if you have broken any of the T&C's during your play through requirement, then the casino have the right to void any winnings that you have made from your bonus.
WHAT IS WAGERING FINAL THOUGHTS
The answer to what is wagering in gambling is pretty straight forward once you have the grasp of it, it's basically just a multiplier that the casino attach to a bonus you have accepted, and you then have to play through the amount that many times. If you do decide to take a bonus and attempt the play through requirement, you should be able to access your progress on your account page. This will tally up all of your play and let you know how much you have left to wager before your balance becomes withdrawable.
If you have any problems with this such as it not updating or not displaying, then contact the support of the casino who will be able to tell you how much of your bonus you have wagered so far.