An income tax slab is simply an income bracket — a range of income — that determines what tax rate applies to that portion of your earnings. India uses a slab system rather than a single flat rate, meaning different portions of your income are taxed at different, progressively higher rates as your income increases, rather than your entire income being taxed at one rate.
For example, under a typical slab structure, the first chunk of your income might be tax-free, the next chunk taxed at 5%, the one after that at 10%, and so on upward. This means someone earning more doesn't suddenly pay a higher rate on their entire income — only the additional income falling into the higher bracket is taxed at that higher rate. Slabs differ between the old and new tax regimes, so it's worth comparing both before filing.