Your marginal tax rate is the rate charged on your highest band of income — the rate your next dollar earned (or last dollar deducted) is taxed at. Because the US uses a progressive bracket system, only the income within each bracket is taxed at that bracket's rate.
Marginal rate is different from your effective rate, which is total tax divided by total income and is always lower. Understanding your marginal rate helps you value a deduction or weigh a Roth-versus-traditional decision.