A marginal tax is a tax applied only to the amount that goes over a certain limit. It is defined by both a rate and the lower limit of the amount to which that rate is applied (though the lower limit often goes unmentioned in discussions of "top marginal tax rate").
If we have a marginal tax rate of, say, 50% on income over $10,000, and no tax below that, then, for example --
- income $10,000 - tax is $0
- income $10,001 - tax is $0.50
- income $10,100 - tax is $50.00
- income $20,000 - tax is $5,000
...and so on.