Form 26AS is statement issued on behalf of the Income Tax Department of the GOI (Government of India) detailing the taxes that are paid on your behalf of by deductors and/ or buyers and confirmation from the banks that such tax has been paid.
Essentially if you have any income for which, tax has been deducted at source (TDS) (for e.g. by your employer when paying your salary or dividend paying company when paying dividends on equity shares held by you) it should reflect here.
Providing Form 26AS, allows us to ascertain the Taxes paid by you via TDS, Self Assessment etc. to claim Tax credit. Income Tax Department will generally allow a taxpayer to claim the tax credit as reflected in Form 26AS.
You can download FORM 26AS from TRACES website.