It's a challenge where you choose a character from either the anime, games or manga and play as them. They can be from any generation, so you can basically play any of the games. For example, I chose Corey from Mewtwo Strikes Back. I named my character Corey in Pokémon Red and captured only the Pokémon that he owns. I completed the games using only those Pokémon. Here's the link if you need more information. There's a rule about playing as Ash, and the character needs a certain amount of Pokémon. It's a really awesome challenge though.