The album is a section of the game where you can view all your series of cards. To enter it, go to the game's Shop and then go to the Gems section, which can be found in the bar at the top of the screen.
Then click on "Get more cards" on the card board, and finally on "Album". There you can see the information of each card (prizes, rarity and description), add cards to your board and share them on your social networks.
The cards come in series of six units. As soon as you get the first 5 cards of each collection, you will unlock the Special Card. This will be the rarest card in the collection and the one with the best prizes.