The Game Genie is a video game peripheral that allows you to make slight variations to the ROM of a video game by entering in various codes from the code book that came with it before you start the game. These codes directly affected areas of the game's programming and usually resulted in small changes that would have a large affect on the game. For example, unlimited lives, always keep a power-up, start on the final level, and so on. Game Genies were released for the NES, SNES, Game Boy, Genesis, and Game Gear, but we only ever had the NES model.
My brother either borrowed a Game Genie from a friend or bought one, because I remember having one in our NES for quite some time. I loved the gold metallic paint which matched the color of The Legend of Zelda cart. We purposely broke the handle off of ours because the Game Genie had an interesting side effect of making the NES much less likely to glitch, and even if we didn't want to cheat, we could bypass the codes.
I spent a fair amount of time trying to make up custom codes. I did this by randomly entering codes into the Game Genie before the game started, writing down the codes I had entered, and then seeing if they made any differences to the game. Anyone who understands how the Game Genie works would realize how terrible a method this is for discovering good codes, but I did find a few interesting codes.
In the early 2000s, I found out that you could enter Game Genie codes into NES emulators, and I spent some time trying to figure out how they work. I eventually figured out what was going on in the ROM, and started making my own codes, and later created a web site for hacking NES games, though, I now use the actual hex codes rather than the encoded Game Genie values.