Slots
A slot machine is a price you can't see. The reels, the sounds and the near misses are all decoration on a program that picks the result the instant you press the button and keeps a set share of everything played through it.
The spin is over before the reels move
Inside every modern slot is a random number generator. When you press spin, it has already picked the result, and the reels are an animation of a decision that's been made. Stopping them early with a second press changes the show and nothing else.
The program decides the result from a table of outcomes set by the game's designer. Some results pay a little, a few pay a lot, most pay nothing, and the weights on each are chosen so that over millions of spins the machine pays back a fixed share of what goes in. That share is the return to player, or RTP. What it keeps is the hold.
In Nevada, Regulation 14.040 sets the floor: a machine has to theoretically pay out at least 75 percent of everything wagered, for each wager available on it. That's a floor, not a typical figure, and other states and tribal compacts set their own. What's legal where you live, and what the minimum is, depends on where you live.
What RTP means in dollars
RTP is a long-run average, and the run is very long. A machine set at 92 percent doesn't hand back $92 of every $100 you bring. It keeps 8 percent of every dollar you put through it, and the money goes round and round.
Say you sit down with $100 and play $1 spins. Your wins go back into the machine, so after 500 spins you may still have money in front of you, but you've wagered $500. At 92 percent, the expected cost of those 500 spins is $40. Play 1,000 spins and it's $80. The hold doesn't care how much you brought. It cares how much you cycled.
That's the number I'd want a slot player to understand above all others. A bookmaker thinks the same way: the hold on a sportsbook is a small cut of a big handle, and a slot is a machine built to maximize handle. The RTP page has the measure in more detail, and house edge compares it game by game.
Volatility
Two machines can both return 92 percent and play nothing alike. One pays small amounts often and rarely anything big. The other goes long stretches with nothing and holds a few huge prizes in its table. The RTP is the same; how it arrives is not. The trade calls the difference volatility.
High volatility is how a machine with a big top prize pays for it: more spins lose so that, now and then, one pays enormously. If you play that kind of game, expect your bankroll to run out before the machine ever reaches its average, because for most players it will. Low volatility gives you more time at the machine for the same money, and more time means more spins, which means more handle. Neither one changes the price.
I'd pick by what you want out of the session. If it's time and entertainment, low volatility. If it's a shot at something large, accept that the shot usually misses.
A machine is never due
The belief I'd most like to kill is that a machine that hasn't paid in a while is about to. Every spin is drawn fresh, and the program has no memory of the last one. Nevada's rule says it outright: a gaming device must not alter any function based on its actual hold percentage, and every possible outcome must be available on each play. So a cold machine isn't loaded and a hot one isn't spent.
The same goes for the near miss, two jackpot symbols and the third just above the line. It feels close. The result was decided by a number, and the number wasn't close to anything.
Players also wonder whether the casino tightens a machine on a Saturday night. I wouldn't spend any energy on it. Whatever the setting, you can't see it from your stool, and the price of the machine is the price. The thing you control is how many times you pay it.
Reading a machine before you play
Online, many games state their RTP in the help or info screen. Read it. If a game won't say, I'd treat that as an answer.
On a casino floor you usually can't see the RTP at all. You can see the bet per spin, which is the number that actually drives your cost, and the pay table, which tells you what a maximum bet buys. Some machines only pay their top prize at maximum bet, and the glass will tell you. The progressive kind, where the top prize grows with play, is covered in jackpots.
Bonus money tied to slot play is a separate sum, and the bonuses page shows how to work out what a rollover really costs.
Setting the session
Slots are the fastest way in a casino to put a lot of money through a small edge. That's not a reason to avoid them. It's a reason to set your number before you start: a budget for the session, and a spin size small enough that the budget lasts as long as you want to play. When you reach the number, cash out what's left rather than playing the credits down to zero.
If the session keeps going past the number you set, that's the thing to look at, more than the machine. The responsible gambling page is where I'd start.