Moving money from a pre-tax retirement account into a Roth, paying tax now for tax-free growth later.
A Roth conversion transfers funds from a Traditional IRA or 401(k) into a Roth, adding the converted amount to that year's taxable income in exchange for tax-free growth and withdrawals afterward. It is most valuable in lower-income years — such as early retirement before RMDs and Social Security — when the conversion is taxed at a lower rate.
Converting part of a Traditional IRA during a low-income gap year fills up a low tax bracket.