The Rumble Pak is an add-on accessory for the Nintendo 64. It is plugged into the expansion slot on the Nintendo 64 controller and when activated, caused the controller to vibrate. It was designed and produced by Nintendo, and was first released as a box-in with the game Star Fox 64. The Pak is powered by two AAA batteries.
In Star Fox 64
The Rumble Pack was used to simulate crashes with solid objects, such as crashing into enemy ships, meteorites, ground, etc, and other things, like enemy fire. It helped popularizing Star Fox 64.