The marginal tax rate is simply the tax rate given in the income tax table. For example, if the table says that people who make up to $4999.99 pay a tax of 10% and that people who make more than $4999.99 but less than $10,000 (for example) pay a tax of 20%, then if your income increases from $4,000 to $5,000, the marginal tax rate you encounter would now be 20%. For a better understanding, notice the word marginal. In economics, this means the 'cost of one additional unit' of something. Thus, the marginal tax rate is the tax on 1 additional dollar of income.
