Card Packs contain a number of useful things. Depending on the type of Card Pack you win or purchase, the contents will vary. Tap on any Card Pack to see the Stadium level, as well as the expected contents of each pack.
When you win a Play 1v1 game, you'll receive a free Card Pack for that Stadium! You can only have four unopened packs at once, and they take time to unlock. Any extras you earn when your Pack holder is full will be discarded.
Packs you win from Special Events or purchase in the Pro Shop open instantly!