Payout percentage is the percentage that a slot game pays out compared to the amount invested in it. If the game is set to 97%, then it will only hold 3% of money that goes into play, and random players will leave with 97% remaining. But this number is never completely accurate, because the payout percentage is only a theoretical number and will be more accurate the larger the sample of spins.
There is another theoretical number that might better explain the payout percentage that a particular game should provide. The expected variance is the number at which the payout percentage may somehow differ from the percentage. If a player launches 1000 spins, he can expect the payout percentage to be in the range of 30-45% of the expected percentage. If there are more spins in the sample, for example 10,000, the deviation will be less, from 10 to 15% depending on the slot machine you are playing. A sample of 10,000,000 will have a deviation of only 0.3 to 0.5% from the expected payout in the game.
This shows you that any session is not accurate at all. Any session can go as you like, you can win or lose, it all depends on luck. So trying to find games that should be paying out or that are getting hotter is a waste of your time. Having a favorite machine that you play frequently does not increase your chances of winning, unless that machine has the highest payout percentage at the moment.
The only thing you can do is trust that the casino is publishing the truth about the long-term payout percentages and that you are choosing the games with the largest you can find. In the long run, playing games with a higher payout percentage will make you a winner more often than just playing any old slot machine. Find the best odds and play these games - your bankroll will thank you in the long run.