You’ve checked in, made it through security, got to the gate, queued and struggled with those overhead lockers and finally settled back into your seat. But then the captain says, “We’re waiting for a slot.”

Slot is an opening or position, as in a time or space, that can be filled or occupied. It’s also a name of a type of casino machine with reels, a central computer and a payline that can pay out winnings to players if symbols line up in the correct sequence.

The first mechanical slot machines were invented in the 19th century by Sittman and Pitt, who created a contraption that used five drums to line up poker cards and allow for a much higher payout than previous machines. But it was Charles Augustus Fey who created the first true electromechanical slot machine in the early sixties with a bottomless hopper and automated payouts that made the side lever obsolete.

Since then, slots have become one of the most popular forms of gambling in casinos and online. They don’t require the same level of strategy or instincts as other casino games like blackjack and poker, but having a good understanding of how they work can help you make better decisions about which ones to play.