Form 26AS is a consolidated annual tax statement, available online through the income tax e-filing portal, that shows every TDS and TCS (tax collected at source) deducted against your PAN by all your employers, banks, or other payers throughout the financial year, along with any advance tax you've paid yourself.
It essentially acts as a tax passbook, letting you cross-check that the tax deducted from your salary, bank interest, or other income actually reaches the government and matches what's mentioned in your Form 16 or other TDS certificates. Reviewing Form 26AS before filing your income tax return is a useful habit, since any mismatch between what you report and what's shown here can lead to notices or delays in processing your return and refund.
It's freely accessible by logging into the income tax e-filing portal, and checking it a few weeks before filing your return is a simple way to catch any TDS mismatches early.