From Bulbapedia, the community-driven Pokémon encyclopedia.

The Sumo Conference is an annual Pokémon Sumo tournament held anually in Rikishii Town. The prize is a King's Rock and a year's supply of Pokémon food. Pokémon entered in the competition must weigh 80kg or more. Only basic physical moves are allowed, and the goal is to eventually throw the opponent out of the ring.