Getting our taxable income down to zero involves the standard deduction and personal exemptions.

For example, if our income is $50k, and DH maxes his 401k ($18k), then our AGI is $32k. Once you subtract the standard deduction for MFJ ($12,600) and 5 exemptions for 2 adults plus 3 kids ($4k each, total $20k) you can see that our taxable income is zero. So our federal tax is zero. There is no point to putting contributions into a tIRA, to lower our AGI further below zero, so we contribute to Roth IRAs instead (tax free going in, tax free coming out).

Whether you can get your taxable income down to zero depends on your details: gross income, 401k deductions, HSA deductions, payroll deductions for things like health insurance, dental, etc., size of family (number of dependents), filing status, itemized vs standard deduction. Then there are credits, some refundable, some not. Nonrefundable credits can only wipe out your tax due, while refundable credits can actually be returned to you if you owe no tax. The refundable credits are what I actually use to fund our Roth IRAs (fully some years, partially now).

You can fund either a Roth IRA, or a traditional one, or any partial combination of both up to a max total of $5500, as long as you are eligible. If you have no income of your own, it is a spousal IRA using your spouse's income. The question is which is better for your tax situation.

The comment about $23,500 to max both out is referring to maxing a 401k @ $18k plus an IRA @ $5500.