Slingo was a game made by a small company that sold pretty good the year it came out, because it had some hidden programs on it that you could use to run a gambling machine with. In states where poker machines were illegal, you could buy one of these and operate it. It had the ability to win free credits, and was set up with games that mimicked card games. So for instance, you might have little cars with numbers on the side of them instead of cards. If you bought 4 credits with a dollar, you might win 40 credits. There was a hidden operator menu, where an operator with a password could remove all the credits from the machine.
So.... say somebody wants to gamble on the machine and it's in a gas station or whatever, they win 400 credits, the operator walks over, knocks the credits off the machine and gives the guy 100 bucks.
Some states changed their laws slightly to make them illegal.