What is the thing that plugs in to the back of a n64 controller

This slot was designed for several different add-ons. First, the controller pak, which allows you to save your game. Second, the rumble pak, which adds vibration to many of the 64's games. Finally, third, it allows you to attach a transfer pak which is used with games like Pokemon Stadium to transfer data from Game Boy Pokemon games to the 64 so you can access special features and characters.