Understand that the IRS does not "reimburse" anything in the sense of the definition of "reimbursement". Assuming this is for a 2019 tax return, line 16 shows your tax liability. Line 19 shows your total tax payments. If line 19 is greater than 16 that means you paid more than required. The difference is refunded to you and is shown on line 21a.
If line 16 is greater than line 19 that means you did not pay the full amount required and the difference is what you owe the IRS. That amount is shown on line 23.
