Trade Tokens allows you to trade cards with your Clanmates and the Trader. Once you reach King level 8, you will be able to use these Tokens. There is one Token under each rarity, and the number of cards you can trade per Token is as the following:
You can see how many Trade Tokens you have, or initiate a trade, in the Clan tab. Tap the "Request Card" or "Trade Cards" button and then the "Trade" tab.
In the "Trade" tab, choose the card you want to receive, and then proceed to select up to four cards you would agree to give away. When you tap "Confirm," your trade will be posted on the Clan chat. Any Clanmate can choose to accept the trade, as long as they have a Token of the same rarity.
Here's where you can find them: