Also called: tax bracket
Your marginal rate is the tax on your next dollar of income; your effective rate is total tax divided by total income. Decisions such as Roth conversions depend on the marginal rate.
Federal income tax is progressive: each bracket's rate applies only to the income inside that bracket, so moving into a higher bracket never raises the tax on income below it.
In retirement the true marginal rate can be higher than the bracket rate, because extra income can also make more Social Security taxable, push capital gains out of the 0% rate, or trigger Medicare surcharges.