A surcharge is an extra charge levied on top of your calculated income tax amount, but unlike cess, it only applies to individuals with relatively high incomes, kicking in once your total income crosses certain specified thresholds. The surcharge rate itself increases in steps as income rises further, meaning very high earners pay a proportionally larger surcharge than moderately high earners.
Surcharge is calculated as a percentage of your income tax (before cess is added), and it exists to make the overall tax system more progressive by asking higher earners to contribute proportionally more. There are marginal relief provisions built in, designed to ensure that a taxpayer's total tax outgo doesn't increase by more than their income increase when they cross a surcharge threshold, avoiding an unfair jump right at the boundary.