If your salary includes HRA and you live in rented accommodation, you can claim an exemption that lowers your taxable income under the old regime. The exempt amount is the least of three figures: the actual HRA received, rent paid minus 10% of basic salary, or 50% of basic salary in a metro (40% elsewhere).
For example, on a ₹40,000 basic salary with ₹20,000 HRA, paying ₹18,000 rent in a metro, the exemption is the lowest of those three formulas — often a few lakh a year.
HRA exemption is only available in the old tax regime, not the new one. If you do not receive HRA but pay rent, you may instead claim a smaller deduction under Section 80GG.