On my Iowa return, I see a number on line 33 "Additional federal tax paid in 2018 for 2017 and prior years .....". Can you explain how this number is calculated?

That would be the liability that was owed toward your 2017 return. 

So, if you filed in March 2018 for the 2017 tax year and you owed, that would be additional federal tax paid.

I had a refund for the 2017 tax year so I cannot figure out why TurboTax is putting a number on line 33. This is not a number I see anywhere on my 2018 return or in the numbers I put into TurboTax.