Ubi Key is a feature that was implemented into a small number of Game Boy Color games. In order to use this feature a key would be found somewhere in a Ubi Key enabled game. Once the key was found it was saved on that users game that it was found.
If the owner of the game found another owner of a Ubi Key enabled game then the owners could then transfer the keys between games using the Game Boy Colors infra-red port. These keys are transferred temporarily and would be erased when the Game Boy Color system was turned off. These bonus key features were mainly bonus levels or mini games.