From Bulbapedia, the community-driven Pokémon encyclopedia.
Card Pop! is a feature in the Pokémon Trading Card Game that resembles Mystery Gift. When the infrared ports of two Game Boy Colors with the game are connected, each player will receive a random card. Card Pop! is the only way a player may receive a Phantom Card (Mew or Venusaur). However, a player cannot Card Pop! with the same person twice.