It refers to the 2018 return. TurboTax is calculating how much, if any, of the refund is taxable.
The total amount of "payments and withholding" includes any payments you made to your state or locality, along with any state or locality withholdings from the previous tax year, including:
- Amounts withheld on your W-2, W-2G, and 1099
- Estimated tax payments for 2018
- Add any taxes paid in 2019 for 2018 (it's common for the fourth quarterly payment to be made in January)
- Overpayment from 2017 applied toward your 2018 return
- Payments made with an extension of time to file
- Payments made when you filed your return
- Refundable credits
You can find most of this info on your 2018 state tax return.
