Do you mean the Pokemon World Tournament?
The Pokémon World Tournament also known as the PWT, is a facility in
Unova in Black 2 and White 2. It is located to the south of Driftveil
City, where the Cold Storage used to be in Pokémon Black and White.
Prominent Trainers from previous games gather here to participate in
battles in an eight person single-elimination tournament. Single
Battles, Double Battles, Triple Battles, and Rotation Battles are held
here. Special Trainers can be downloaded via Nintendo Wi-Fi
Connection. It also contains both the move relearner and Move Deleter,
as they are no longer located at Mistralton City.- Bulbapedia
So In other words it's like the Pokemon Battle Subway, except instead of 7 battles there are 8 battles. You choose either if you want Single, Double, Triple, And Rotation Battles. Also if you want you can download something to the Tournament using Wi-Fi which makes you battle against Stronger Trainers. After you defeat the 8 battles you will get BP (Battle Points) and you can use the BP to buy items. You can also use Rental Pokemon if you want.
And after you have defeated all the Unova Gym Leaders you can choose which region of Pokemon Gym Leaders/Trainers you want to battle.