The Jet Pack is a machine and one of the many inventions created by Sandy.
The Jet Pack only appears in the episode "Spy Buddies" and resembles a brown book bag with a little cone-shaped rocket on the bottom of it.
Role in the episode
When SpongeBob and Patrick want to become spies, they receive many gadgets from Sandy. The jet pack is used by SpongeBob throughout the episode to consistently follow Plankton around. It burnt Plankton twice when SpongeBob got too close to him.