What is Marginal tax rate?
The rate applied to your next unit of income, which is the rate of the highest band you have reached and not the rate on everything you earn.
It is the number to use for any decision about earning or deducting more, because an extra rupee is taxed here and a deduction removes income from here. It says almost nothing about the size of the bill.
Not to be confused with Effective tax rate
The marginal rate applies to the last unit of income; the effective rate is the average across all of it. Under any progressive schedule with a tax-free band the effective rate is always the lower of the two, often by more than ten points.