Do you own company stock in your 401k?

If so, when you change jobs or retire-that stock is generally eligible to be rolled over to an IRA account.  You'll probably have the option to roll over the cash or the stock in kind.  The plan administrator is typically tracking the cost basis of that stock (the original purchase price).  If the stock has a very low cost basis then taking advantage of NUA or net unrealized appreciation may be advantageous for you.  It allows you to move stock into a taxable account, pay taxes on the original basis and more favorable capital gains rates on the difference between that basis and the current value.  Remember, distributions from the IRA account are always taxed as ordinary income.  It's a complicated topic and you'll want to talk with your tax advisor.

Ed Slott explains some of the details.