The term "source of wealth" is still not very common among players. In short, the online casinos want (or rather have) to check in this way whether it is actually one's own wealth that has been wagered in the game. The "average" recreational Player will almost certainly not have to undergo a "SoW" check. However, the probability increases when playing with significantly higher stakes. Accordingly, high rollers will certainly have to provide such proof sooner or later.
The Online Casino primarily wants to make sure that the player has only played with his own funds. For this reason, some of the following information, documents or proofs are usually requested:
Normally, a "Source of Wealth" proof is requested at the latest when a larger amount is deposited. However, some online casinos request the relevant proof only when the player wants to make a (larger) withdrawal. In some cases, online casinos also try to drag out requested payouts by asking several times for the "SoW" proof, which is of course not in the interest of player protection.
Basically, however, especially players with high deposits (especially high rollers) have to put up with such a process. Those who refuse to provide the required documents and proof Risk being denied payouts of Casino wins. After all, online casinos want users to play only with the assets that belong to them.