What does "was your 2023 tax liability zero?" mean?

It depends. Below is an IRS FAQ about it.

Question: What is meant by "no tax liability" in the exceptions to the estimated tax penalty?

Answer: You had no tax liability for the prior year if your total tax was zero or you didn't have to file an income tax return.

Your total tax was zero if the line labeled "total tax" on Form 1040, U.S. Individual Income Tax Return or Form 1040-SR, U.S Tax Return for Seniors was zero.

You may not have had to file an income tax return for the prior tax year if your gross income was below a certain threshold. See Who Must File in Publication 501, Dependents, Standard Deduction and Filing Information, for information on filing requirements for most taxpayers.

For this exception to apply, two additional requirements must be met:

• Your prior tax year was a taxable year of 12 months.
• You were a citizen or resident of the United States throughout your prior tax year.