The tax rate applied to your next dollar of income.
The marginal tax rate is the rate charged on your highest band of income under a progressive system, where different slices are taxed at increasing rates. It is not the rate on your whole income — that is the effective rate. Knowing your marginal rate guides decisions like whether a deduction or a Roth contribution is worthwhile.
Being "in the 24% bracket" means only the income above that bracket's threshold is taxed at 24%.