These are alphanumeric strings entered into a Game Genie device, which then modifies the underlying data of the Pokmon Crystal game. For instance, a code might alter the properties of encountered wild Pokmon, the number of items held in the player’s inventory, or even the way the game calculates experience points.
The appeal of using these lies in the capacity to customize the gaming experience, circumventing typical gameplay constraints. This allows players to quickly acquire rare items, boost character stats, or encounter specific Pokmon not normally available early in the game. Historically, such code systems offered a way to enhance replay value and experiment with otherwise inaccessible game mechanics before more sophisticated modification methods were readily available.