Cards are collectible items that you can obtain in the machine room in exchange for gems. To enter the room, you must press the "Get more cards" button on the gift board, which you can enter from the game's Shop, selecting the "Gems" option.
Each card will give you a special prize by placing it on the game's gift board every certain period of time, and both the prize and the time to wait will depend on each particular card.
On the board, you can place a maximum of three cards and our recommendation is that you select the cards that give you the best prizes in the shortest amount of time possible.
All the cards you get will be included in your album, which you can access by going to Menu > View profile > Album, or Shop > Gems > Album.