There is one category of assets/funds in IRA's or 401(k)'s that meet this description: The ROI in Roth 401(k) or Roth IRA. I believe that's what the article is referring to.
An example: you contribute $5,000 to a Roth IRA this year (that's the maximum, if you are under 50). You will have paid income taxes on that contribution, because only earned income is eligible.
However, if you don't withdraw those funds for 20 years, and you get an average ROI of 7% per year (which isn't unreasonable), that original contribution will grow to nearly $20,000.
On withdrawal from the Roth IRA, you won't pay income taxes on any of it, as long as you don't withdraw it before the year you turn 59-1/2. That means that nearly $15,000 is never subject to income tax. Furthermore, there is no minimum required distribution, so you don't have to withdraw from it at all. Then, you can bequeath your Roth IRA to a spouse, child, grandchild, etc. If it is someone besides your spouse, they will have to start withdrawing from it within 5 years after your death, but the minimum required distribution rules apply and they can spread it over their entire remaining lifetime. And except for your original contributions, it will all be tax-free, aside from any estate tax that may have been assessed.