It’s regular income tax on income you shielded from taxes when you put it into the 401k and any un-taxed matching funds that were added by your employer.
A theoretical advantage was that you’ll be in a lower tax bracket after retirement. That certainly applies to some folks, but things may change over time.
OK, then I think I’d probably only get hit with short- or long-term capital gains if I close my IRA. It is funded with 100% non-deductible contributions.
I plan to keep the 401(k) as that is way too big and will make me one of those ‘private jet flying’ rich people O’Bumma always talks about.