On Form 4868:

* Total 2018 Federal Taxes Owed means your total estimated tax liability for 2018

* Total 2018 Payments will include your federal withholding plus any estimated tax payments

* If your estimated tax liability for 2018 is less than your payments, this is your balance due, which should be paid with your extension.

I am including a link to Form 4868 & the instructions for your reference: https://www.irs.gov/pub/irs-pdf/f4868.pdf.