Store credit is an amount that is refunded to a customer account. You can use their store credit to pay for purchases, 

To apply store credit to your current purchase. First please log in to your customer account with the same email the store credit was applied to before beginning the checkout process.

During the Review & Payments step of the checkout process, elect to “Use Store Credit” as a payment option. The available balance is shown in parentheses If the available balance is greater than the order grand total, the other payment methods disappear.

The amount of store credit that is applied to the order appears with the order totals, and is subtracted from the grand total.  The available balance is adjusted when the order is placed.