Bingo (Class II) machines came out of federal guidelines that made it much easier for tribes to launch Bingo operations. The regulations allowed for the game of Bingo to be played, and also allowed for aids designed to support the game of Bingo. Getting approval to offer Bingo under Class II rules is a much lower bar to clear than a compact with a given state to offer Vegas-style slots that are Class III, even though they are ultimately what most are familiar with, so some decided to see if there was a way to get around the Class II limitations through technology.
The Seminole Tribe in Florida is given a lot of credit for investing a lot of time and resources to develop the technology that still powers a lot of Class II gaming today. VGT, now owned by Aristocrat, also invested a lot of time and energy popularizing Class II options in states like Oklahoma. The goal was to take the popular large bingo halls and bring other casino-style games in while still qualifying as a bingo "aid," and the Bingo (Class II) machine was born.
At its core is a machine that looks like any other slot machine, but shows a Bingo card somewhere on the screen. When you make a wager, you're actually joining a Bingo game with others who hit the button within a few milliseconds. Most systems only require at least two players to take part. The game is played out rapidly and in real time, and the machine then renders a game outcome that either shows a winning combination or not, based on the outcome of the game that rendered behind the scenes.
There are interim jackpot combinations that can be hit in some games, just like in actual games of Bingo, which can then contribute to bonus outcomes on the game as well. For instance, many classic VGT games can have a red screen that can trigger and offer bonus spins; these bonus payouts are part of the bingo game and are registered in a fun and entertaining way.