Store credits are spending currency that you can use to make purchases on freyja.com. If you have store credits to spend, you will see it appear on the checkout page just above the payment options block. You need to be logged in to access your store credits balance.
If you have more store credits in your account than is the order value in your cart, you will need to pay for the order entirely via store credits. If you can only partially pay for the order with store credits, you can choose to pay for the balance via any of the other payment methods mentioned.