Marginal Rate or the marginal tax rate refers to the percentage of your taxable income above a certain defined threshold. It is only applied for each tax bracket that you qualify for. The more income you make, the more taxes you owe as it is simply a percentage of your actual income.
Example: A person earning in the bracket of 250000 to 500000 would have to pay 5% taxes only on the earnings between 250000 to 500000, which means the first 250000 is not taxable.