From RWP Wiki
A GameShark is a hacking device used on the N64. You would plug in a game on the top of the GameShark, and then put the GameShark into the N64. You could then put in codes which would alter the game, such as being able to walk through walls and invincibility. In Banjo-Kazooie , the GameShark used to be the only way to unlock Stop n' Swap, until the finding of the Sandcastle codes.