Some technology gets outmoded over time. One such example is the Coin Hopper, which was necessary when machines accepted and paid out coins. The Coin Hopper houses the money available to pay a player if they win on a given spin.
When a winning combination would be triggered, coins would be pulled from the Coin Hopper to be dropped for the player to collect. As such, the Coin Hopper played a critical role in the slot machine’s inner workings, and tended to be a significant space consumer inside the machine to house the money.
Coin Hoppers would need to be monitored and filled periodically by the casino to ensure that the game could pay out coins when win would take place. Before TITO systems existed, this was a necessary way to ensure a player was compensated without having to wait a lengthy period of time.
Nowadays they’re nearly unnecessary, except for machines at casinos where coins are still accepted. This is probably a good thing, given the coin shortage going on in the country right now.
In the event a Coin Hopper would run out of coins before it pays out the full prize, this is known as a short pay. The slot machine would trigger an error, which would bring out a slot attendant to either pay by hand the remaining amount, or get the Coin Hopper refilled to complete payment.
Some older machines out there may still have Coin Hoppers inside, or software that can detect and work with Coin Hoppers, even though they no longer accept coins; some hobbyists will deal with Coin Hopper errors on home machines even though the machine isn’t going to actually pay out coins.
So, you might hear the phrase now and then, but it’s a largely retired aspect of a slot machine given today’s modern technology.