GSTR-1 is the statement of outward supplies — essentially your sales — for a period. It reports B2B and B2C invoices, credit and debit notes, exports and an HSN summary. The data a buyer sees in their GSTR-2B flows from suppliers’ GSTR-1 filings.
It is filed monthly, or quarterly under the QRMP scheme for smaller taxpayers. Accurate GSTR-1 filing is what lets your customers claim their input tax credit.