Cubeb berries are the unripe fruits of the Indonesian piper cubeba plant which is in the pepper family. The berries have a spicy flavor with hints of allspice. Author Alan Davidson describes the flavor as "hot, camphorous, peppermint taste".
Cubeb is best ground fresh and used in spicy Indonesian dishes or you could use this as a substitute for ground black pepper. The berries are used in Asia for medicinal purposes. Some consider this an "outdated" spice but it has gained new popularity as an exotic ingredient.
You're probably not going to find this in your "average" grocery store. You can them in some specialty food stores as well as online at Amazon.com: Cubeb Berries.
If you can't find cubeb berries substitute equal amounts of: