You can redeem codes in the Kards shop, for example to get card packs.
How to redeem codes
On the main UI screen, click the Shop Icon.
Then, on the Shop screen, click the "Packs" tab at the top and the "Redeem" button at the bottom.
In the popup box, enter the redeem code and click "Confirm".
This will redeem the code and add the contents directly to your Kards account.