Look at Schedule 2 to see if your return is generating additional taxes. Key numbers on Form 1040:
Line 15 - Taxable Income
Line 24 - Total Tax - - this will include additional taxes such as Self-Employment tax, AMT, Excess Premium Tax Credits, etc.
Line 25 - Withholdings
Line 33 - Payments including Credits
Line 34 - Overpayments
Line 37 - Tax owed