Modern casino slots are controlled by computer chips and software, and the win percentages are preprogrammed into the software.

Casino slots have Random Number Generators (RNG) which are continually generating combinations, even when the equipment isn't being played.

Running concurrently with the RNG of the casino great slot machine is the payout percentage.

The payout percentage controls how much the slot machine will payout, for example the payout percentage may very well be 90%, meaning the slot game will payout 90% of all of the bets played and the casino only gets to keep 10%.

This is not to say that every time you bet 10 credits you shall receive 9 in return - where will be the fun and excitement in that? Instead the casino slot machine game will not pay out for a while, and after that suddenly go on a hot streak and also you might hit a big jackpot.