The payout percentage of a slot machine is The expected paid amounts generated by a slot machine divided by the money input to the machine over the life of the machine.
What it really means is that a 90% payback slot can be expected to return about 90% of the money it has taken back to customers over its lifetime. The key phrase here is 'over its lifetime,' because slot payouts cannot be determined in short time spans. are hard to measure accurately in short time frames. Usually it takes at least a month or two to get a good reading if the slot is new, or perhaps a single month if the slot has still been on the floor awhile.
What it really means to YOU is that a 90% machine is better in terms of odds than an 80% machine because the 90% machine will end up paying more money over the course of its entire life... but it doesn't mean that 90% of the people who play will win. The 90% isn't talking about spins or people. It's talking about money. If 99 people each put a dollar in the machine and win nothing, then the 100th person walks up, puts in a dollar, and wins $90, then machine has a 90% payback. 90% of all the money put in the machine, in this case $100 was put in and $90, was paid back eventually. But again, it is an estimated percentage over a LONG PERIOD OF TIME, and doesn't mean every 100 dollars entered will pay 90. It could be more or less.