26AS is a consolidated statement showing various taxes that are deducted from your income by your employer, bank, or your tenant. It shows how much tax has been received by the government by way of TDS deposited by the deductor (employer, bank), advance tax or any self-assessment tax that has been paid, etc.
It also contains the details of income tax refunds that you might have received. It also shows AIR (Annual Information Return) transaction details, which might have been filed by your bank in case you have entered into some specified transaction. You must match tax payments and TDS deducted with 26AS before filing your ITR to get a tax credit as the tax credit is given only on the items appearing in our 26AS.