Not many people have absolutely ZERO idea what a slot machine is https://theonlinecasinoslots.com/. But we wanted to answer this question because we think it provides some useful context. Slot machines are those big boxy video game like gadgets on the casino floor with flashing lights at the top. They're the noisiest machine on the casino floor. Their cases are brightly-colored, densely-decorated, covered in symbols and numbers. Slot machines have spinning reels with symbols on them, and the spinning is just a little hypnotic. Slot machines are noisy games (increasingly computer-controlled) that offer long odds on some large payouts.
A plain definition of the phrase "slot machine" might look something like this: a slot machine is any device that allows real-money gambling, gives players 100% control of its operations, pays out winnings based on the series of symbols that appears on spinning reels. A long time ago, a slot machine was a coin-operated mechanical gadget with small prizes and just a few symbols. Back then, a pull of a lever would activate the game, set three reels spinning, and wins were automatically determined and paid out into a hoper.
That's all changed.
Slot machines these days may not have a lever to pull; having replaced it with a "spin" button or a touchscreen. They probably don't accept coins. Very few casinos host "old-school" machines that accept coins or require a real lever to start the game. You won't usually hear coins spilling into hoppers anymore, either. These days, prizes are tallied and paid out electronically, the same way the players place their bets.