The cube (キューブ, Kyūbu) is a vehicle that can travel freely any where in the universe and between other universes. It is also known as a hexahedron as specified by Whis in episode 32 of the dub.
The cube is the vehicle of the gods. It can be used to travel anywhere in the universe and can also go to other universes. Champa has a cube and it is promised to Hit by Vados if he wins the tournament. This cube is also used to transport Team Universe 6 and their guests to the Nameless Planet. Whis uses another cube to transport Team Universe 7 and their guests to Beerus' planet to pick up Beerus and Monaka and then the Nameless Planet for the tournament.
During the Universes 6 and 7 Gods of Destruction Selection Martial Arts Competition, Hit has lost interest in obtaining the cube because of his entertaining fight with Goku, so he loses his fight with Monaka on purpose. As a result, Team Universe 6 loses the tournament, declaring Team Universe 7 the victors.