Your tax liability is the total amount of tax on your income minus any non-refundable credits such as child tax credit, saver’s credit, dependent care credit to name a few. So basically, just because you got a refund last year does not mean your tax liability was 0. It is the actual tax which you paid last year.
To see your 2020 Tax Liability, refer to your prior years tax return (Form 1040) and look at line 24.
