In the options menu, there is a special option that allows you to put in a password. These passwords will unlock special extra stages across the game. These stages are visible beforehand but are shown with a lock by them so you cannot access them without the password. These stages have special Pokémon within them that cannot be obtained elsewhere in the game.
There are only four stages obtained via password and once you have redeemed all four, then the password option shall disappear forever. There is also a special Medal for accessing passwords.