Cess is an additional tax charged on top of your already-calculated income tax amount, earmarked for a specific government purpose rather than going into the general pool of tax revenue. In India, the most common one taxpayers encounter is the Health and Education Cess, currently levied at 4% of your total income tax (including any surcharge), and used to fund health and education initiatives.
Unlike a surcharge, which only kicks in above certain income thresholds, cess applies universally to essentially all taxpayers who have a tax liability, regardless of income level. It's automatically calculated and added when your tax liability is computed, so you don't need to calculate it separately — but it does mean your final tax payable is always a bit higher than the raw slab-rate calculation alone would suggest.