"The credit is fully refundable because the Eligible Employer may get a refund if the amount of the credit is more than certain federal employment taxes the Eligible Employer owes. That is, if for any calendar quarter the amount of the credit the Eligible Employer is entitled to exceeds the employer’s share of the social security tax on all wages (or on all compensation for employers subject to RRTA) paid to all employees, then the excess is treated as an overpayment and refunded to the employer under sections 6402(a) and 6413(a) of the Internal Revenue Code (the “Code”). Consistent with its treatment as an overpayment, the excess will be applied to offset any remaining tax liability on the employment tax return and the amount of any remaining excess will be reflected as an overpayment on the return. Like other overpayments of federal taxes, the overpayment will be subject to offset under section 6402(a) of the Code prior to being refunded to the employer."

Source: IRS - ERC General Info FAQs.