The jetpack is a consumable object similar to the coin and the slowatch. After collecting it, a player becomes able to fly by holding the jump button for the rest of the round. It is slightly larger than 1x1 blocks and has a weight of 1. It's part of the Sciency Stuff category.
In addition to pushing the player upwards, the jetpack will also always push the character in the direction they're facing so it's not possible to fly straight up.