EPROM, or Erasable Programmable Read Only Memory, is like the brain of the slot machine. This cheeky little microchip determines the theoretical return of the game - basically, how much of the money that gets put into the machine will get paid back out over time.
Now, if the casino honcho, the slot manager, wants to tweak the return of a slot machine, all they gotta do is replace this EPROM chip. But don't get the idea that they're doing this willy-nilly! Most places have regulations that mean they gotta fill out a bunch of paperwork and suchlike before they make the switch.
So next time you're feeding your pennies into the one-armed bandit, just remember - it's the EPROM that's spinning the reels, not Lady Luck!