The Trading Card Kiosk is a feature that allows you to trade repeated cards for gems. To enter the Kiosk, go to the Machine Room by tapping on 'Get more cards' on the board of the main screen. Once there, you will see the buttons 'Album' and 'Trade' at the bottom of the screen. If you tap on 'Trade', you will be redirected to the Trading Card Kiosk.
You will be able to see the number of gems and repeated cards you have on the upper part of your screen. If you have more than 99 repeated cards, it will be displayed as 99+.
Every 5 repeated cards, the Kiosk will give you a gem and you will have the possibility to trade them 3 times every 4 hours. Once you make the three trades, the Kiosk will show a timer that will start a countdown till you can trade again.