The Sealed Ball is a big brown cocoon (purple in original manga) with vein-like bulges on it. When it opens, it splits in half shooting smoke out.
The cocoon was hidden on Earth by Bibidi. When his son Babidi found it, he took it to the bottom of his spaceship. After Spopovich and Yamu drained the energy of Gohan at the World Martial Arts Tournament, Babidi almost had enough energy to revive Majin Buu. Using the warriors of his spaceship, he gained sufficient energy from the Dragon Team, and the deadly Majin was awakened.