The Tremor Pak is a third-party version of the Nintendo 64's official Rumble Pak, a device designed to vibrate the N64 controller during selected games to enhance realism. For example, the Tremor Pak might cause the controller to vibrate when a gun is fired or when the player takes damage.
The Tremor Pak uses AA batteries, causing it to be heavier than the Rumble Pak, which uses AAA batteries. Unlike the Rumble Pak, the Tremor Pak has two levels of vibration.

Source: Wikipedia, "Tremor Pak", available under the CC-BY-SA License.