A chargeback is fought by the merchant sending in the documents that has been requested by the Issuing Bank. The documents typically are receipts, customer signature, and a written explanation.
The merchant will send the documents to the Issuing Bank, once the Issuer receives the documents the cardholder will have the opportunity to review the documents. The Issuing Bank then can decide if additional documents are required or if the chargeback was found in favor of the merchant or cardholder.
A chargeback is when a cardholder does not agree with certain transaction. There are several reason codes that a cardholder can file under. Detailed Reason Code information is listed on Visa's Chargeback Management Guidelines for Merchants (PDF).