What is it?
If you have a Traditional IRA, you can convert part or all of the account to a Roth IRA. You pay taxes as if you withdrew the entire amount converted, but without any penalty for early withdrawal; in return, the money will grow tax-free in the Roth IRA, and you will not pay any tax on that withdrawal if you meet the Roth IRA distribution rules.
Why does it matter?
The benefit of a Roth conversion depends on your current tax bracket as well as your expected tax bracket. If your current tax bracket is lower than it will be in the future, it may make sense to do the conversion now. Paying less taxes means you get to keep more of your money.
How do people get it wrong?
Retirees may not realize their sources of income may increase the later years of retirement and suddenly push them into a higher tax bracket. For example, if you delay Social Security until after 70 and you have to start taking the Required Minimum Distribution (RMD) from your traditional IRA at 70.5, your income in retirement may now exceed the level of income you had in the earlier years of retirement. It would have been a missed opportunity to convert to a Roth in the years the income bracket was lower.