The Voodoo root was an item that could be used to make Root Beer that can kill ghosts.
The Secret of Monkey Island[]
The root, found on Monkey Island was very rare, and LeChuck, fearing its power, took the only last specimen in the crate below the deck of his ghost ship. Having been told so by the Cannibals, Guybrush Threepwood went there to find the root and took it back to the Cannibals, who made it into Voodoo Root Beer.
Guybrush later lost it while fighting with LeChuck, but made use of a root beer from the Grog Machine. This seems to suggest that the root was not as rare as the Cannibals claimed, or that any root beer could have the same effect.
Tales of Monkey Island[]
In the start of Chapter 1, you need to use the voodoo root on the Cursed Cutlass of Kaflu. You find it in the monkey coffin, but have to make some more using a root and near grog. Elaine Marley also uses Voodoo Root Beer on Guybrush to send him back to the Crossroads.