Section 80D allows you to claim a tax deduction for health insurance premiums paid — for yourself, your spouse, your children, and separately for your parents — reducing your taxable income under the Old Tax Regime. The deduction limits differ depending on age; a higher limit applies if you or your parents are senior citizens, since their premiums tend to be higher too.
This deduction directly encourages having adequate health insurance cover, which is important given how quickly medical costs can add up. It's worth noting that 80D is separate from and in addition to the Section 80C limit, so premiums paid for health insurance don't eat into your 80C investment room. Like most such deductions, it's only available if you choose to file under the Old Tax Regime rather than the New one.