A keg is an item used in the The Great Brain Robbery quest. It is filled with gunpowder and is used to blow up a door open to the monastery on Harmony Island in order to get inside.
During the quest, four kegs spawn on the upper level of the zombie pirate's ship on Harmony Island, but they cannot be picked up, prompting the message You already have a barrel of gunpowder. You do not need another." Even Telekinetic Grab will not work, resulting in the message The barrel is too unstable to telegrab.