The E-Reader was a peripheral released for the Game Boy Advance in 2002 that unlocks special content for certain games. Content in includes mini-games, different worlds, or full and complete NES games. The content was added or unlocked by purchasing e-Reader cards and swiping them in.
Among others, Pokémon Battle Arena E-Reader cards and Animal Crossing Cards were released. The Pokémon battle Arena cards allowed you to fight a large amount of predetermined battles for Pokémon Ruby and Sapphire Version for the Game Boy Advance whereas the Animal Crossing E-Reader Cards were used for the GameCube version of Animal Crossing and unlocked gifts from your Animal Crossing residents.