2016 tax liability is the amount of tax due on your 2016 income, depending on which tax bracket and rate you fell in. Although you may have had tax liability, you may not have had to pay the IRS when you filed your taxes due to credits and prior payments in the form of federal withholding.
Your federal tax liability amount can be found on Form 1040 (line 63), Form 1040A (line 39), and Form 1040EZ (line 12).