What is a required minimum distribution?
Tax-deferred retirement accounts can't defer taxes forever. Once you reach the age set by current law, the IRS requires you to withdraw a calculated minimum each year, based on your account balance and life-expectancy tables, and pay ordinary income tax on it.
The amount changes annually as your balance and age change. Your custodian can tell you each year's figure, but the responsibility to take it is yours.
Why do RMDs need special planning in a metals IRA?
Metal isn't divisible at the push of a button. If your RMD is smaller than your smallest bar or coin, you can't shave off an exact slice, you either sell enough metal to raise the cash or take a whole item as an in-kind distribution.
That makes liquidity planning part of owning metals in an IRA: hold some easily sellable product, and decide each year whether the RMD comes out as cash or as metal shipped to you, which is a taxable distribution either way.
How do RMDs show up in practice?
Each year your custodian reports your balance, the RMD is calculated, and you instruct the custodian to sell metal for cash or distribute metal in kind. Missing the deadline carries a steep excise tax on the amount not taken, one of the harsher penalties in the code.