Tax Refund Notice Letter(국세환급금 통지서)
This is the notice letter for your tax refund and it'll be refunded sooner or later to your bank account registered at the tax office, if there was no bank account no. on this letter, you gotta go to a post office with your ID in person to get paid by cash. And you'd better to register your bank account for the future refund.
If you had an unpaid tax bill, NTS will deduct it from this refund amount with a separate notice letter to do so.