A banker's acceptance (BA) is a bank-guaranteed payment promise used mainly in international trade. The bank stamps "accepted" on a time draft (a promise to pay at a future date), guaranteeing payment. If the buyer defaults, the bank pays.
Here is a typical scenario: A US importer buys goods from a Japanese exporter. The importer's bank issues a BA promising to pay $500,000 in 90 days. The exporter gets a guaranteed payment instrument. They can hold it until maturity or sell it at a discount in the secondary market for immediate cash.
Banker's acceptances are considered very safe because they carry the credit of the issuing bank, not just the buyer. They trade in money markets at rates close to Treasury bills. BAs have been used in trade finance for centuries — the concept dates back to medieval European merchant banking.