Fee credit is one of the ways we say thanks to our customers. If some has been added to your TransferWise profile, you'll see the amount next to "fee credit" on your "Transfers" page.
The amount of your fee credit will be deducted from the fee of the next transfer you set up. If you have some left over after it's complete, your remaining fee credit will be applied to the fee of your next transfer as well.
See also: How does a first payment discount work?
Please note: In case you have both a transfer discount and fee credit on your TransferWise profile, the transfer discount will be applied first. In case your transfer value exceeds the value of the discount voucher (or equivalent in your sending currency), your fee credit will be used to cover the remaining fee.