The Old Tax Regime is India's original income tax structure, offering relatively wider, higher tax-free thresholds combined with a large number of available deductions and exemptions — such as Section 80C investments, HRA, and home loan interest — that can significantly reduce your taxable income if you actually use them.
This regime tends to work out better for people who have meaningful deductions to claim, such as those paying rent and claiming HRA, repaying a home loan, or investing regularly in tax-saving instruments like PPF, ELSS, or life insurance. If you don't have many such deductions, the old regime's higher slab rates can end up costing you more tax than the New Tax Regime, so it's worth comparing both every year using an actual calculation rather than assuming.
If you're unsure which regime suits you, it's worth running the numbers under both before filing your return each year, since your ideal choice can change if your deductions or income change from one year to the next.