Skimming is a term that usually refers to an illegal activity. It is used in brick-and-mortar casinos as well as in everyday life, and it most often refers to stealing money from someone.
If you hear that someone was “skimming off the top,” they were taking a portion of the business’ money for themselves while reporting a lower total.
In the world of casinos, the term is used to refer to a gambling establishment that uses a third party not employed by the casino to carry out the money and thus evade taxes. For example, casinos might rig games and adjust the winning outcome to control who is going to win. As a result, they would pay out the winning amount to a person who’s in cahoots with the casino, technically deducting that sum from their overall profit.
There are numerous ways for casinos or anyone else to skim, but the gist of the word is still the same.