The Trading Card Kiosk is a feature that allows you to exchange your repeated cards for gems. To access it, you'll have to enter the game's Shop, select the gem icon at the top of the screen, and tap on the "Get more cards" button on the Prize Board.
Once you do so, you will find the "Album" and "Trade" buttons at the bottom and, by tapping the latter, you will be redirected to the Trading Card Kiosk. There, you will see your current gem balance and the number of repeated cards you have.
The kiosk will provide you one gem for every 5 repeated cards. In case you own more than 99, your amount will be displayed as 99+.
You will be able to make 3 trades every 4 hours, and once you use them, you will see a countdown until you are able to trade again.