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.