Tax has to be paid on the taxable income. Tax which has to be paid can be either a positive number or zero. A tax credit is a credit which reduces your tax amount. A refundable tax credit is something which reduces your tax to 0 and then goes beyond it.
If the amount of refundable credit > tax owed, the person would actually receive a refund of the difference.
If tax owed is X and the tax credit is Y. Given Y>X then
Y-X is the refund a person would get from the government. This is true even if X was already 0 to begin with this mean the refundable credit would Y.