I have federal refund for this year ('18) and had refunds in past years also.
Why is Turbotax suggesting I prepay estimated taxes for 2019. Don't quite get it
If you are receiving a refund or low tax due and TurboTax prepared the vouchers then your tax liability was probably reduced by credits. Your Tax Liability is the amount on 1040 line 44. It is the tax you owe on your income. But you might qualify for credits that can reduce your tax due or even give you a refund. Credits are on lines 47-55. Excuse me, there are new schedules and lines for 2018.
You can ignore them. The estimated payments are optional to pay. They are estimates for next year. Turbo Tax doesn't want you to owe next year. They might have printed out if you got a one time large income this year. Like if you took a IRA or 401K distribution. They DO NOT get sent to the IRS or state with your return.