The Transfer Pak is a Nintendo 64 accessory that allows the transfer of data from a Game Boy game. The Transfer Pak fits onto the expansion port of the N64 controller. The hardware is limited to the transfer of data; it is incapable of playing Game Boy games on the Nintendo 64.
The Transfer Pak was included with the game Pokémon Stadium in North America, as the game's main feature was importing Pokémon from Pokémon Red, Pokémon Blue and Pokémon Yellow. Pokémon Stadium included a feature called "GB Tower" for playing Pokémon Red, Blue, and Yellow on the N64 via a built-in Game Boy emulator (which included the unlockable "Doduo" and "Dodrio" modes which would speed up the game's processor by factors of two and three, respectively).