Rather than collecting all tax at year-end, the government requires certain payers to withhold a slice upfront. Your employer deducts TDS from salary; a bank deducts it on fixed deposit interest above ₹40,000 a year (₹50,000 for seniors).
The deducted amount is credited against your total tax liability. You see it in your Form 26AS and Annual Information Statement, and you claim it when filing your return.
For example, if a bank deducts ₹5,000 TDS on your FD interest but your actual tax on it is only ₹2,000, you claim a ₹3,000 refund when you file. If no tax is due, you can submit Form 15G or 15H to avoid TDS altogether.