A nonrefundable credit is a tax credit that reduces the amount of taxes you owe. If you do not owe any money it does not directly benefit you. If you would have owed taxes but they were covered by your withholdings from your job, a nonrefundable credit may allow you to get that money back. These credits also make sure that your refundable tax credits like the Child Tax Credit and Earned Income Credits are not used to pay for your taxes but are fully sent back to you as a refund.
