When the casino gives you free spins or any other money bonus, they can`t just let you take this out right away. That would result in great losses, even though the free spins are free, you must earn your winnings before you can turn the bonus into real money you can withdraw to your bank account.
Wagering requirements are the times you must turn over your bonus money or winnings from free spins before they go from being a bonus to becoming real money you can withdraw.
Let`s say you win $40 dollars on your no deposit free spin bonus. If the bonus has a 30x wagering requirement, you must play 40 x 30 = $1200 to fulfill the requirement. The amount you have on your account when you have played for 1200 dollars with your initial 40 euro is now yours to do with as you like, it can be played at any game on the casino, including the live casino or you could withdraw the money.
The wagering requirement for most deposit bonuses are between 30-40x. For no deposit bonuses, it is usually quite a bit higher, if you are lucky you will manage to find one where the requirement is only 40-50x, but often it is 50-70x or even higher.
If it sounds like a trick, that you can't possibly fulfill the wagering requirements, you forget about the casino games RTP percentage. The return to player % should be at least 95%, most of the games at an online casino will have an RTP between 95-98%. This makes fulfilling the play-through requirement doable in many cases.
An example of this can be a 50x wagering requirement on your no deposit bonus casino free spins. On the 30 free spins you win 17 dollars. Now you have to fulfill a 850 dollar wagering requirement. With some luck and an early and good bonus or free spin feature, you can win over 100 euro. Now you can play the long game and let the high volatility games with a good RTP% work for you. With some more luck and patience you could end up with hundreds of dollars when the requirements have been fulfilled.