The Game Genie for the original Game Boy was a plastic accessory that plugged into the console, with the game cartridge then plugging into the Game Genie. It acted as a middleman, allowing players to input special codes from a booklet to temporarily change game data and enable cheats like extra lives or invincibility. The codes worked by intercepting and modifying specific memory bytes as data traveled from the cartridge to the Game Boy, offering a way to customize and enhance gameplay for a variety of popular titles.