From the introduction:
The world of Dragon Ball, a place of mystical energies, epic battles, aliens, demons, gods and more. In Age 784, 10 years after Majin Buu was defeated was defeated by the Z fighters, a mysterious ship appeared on Earth. This ship was from another universe, piloted by the Vargas. These small, bird-like aliens had discovered how to create portals to alternate universes. Whilst deciding that this technology should be sealed away because of how dangerous some universes were, they believed that, for one time and one time only, a tournament should be held.