A chargeback is a return of money on a card payment started by the card-issuing bank after the cardholder has disputed the transaction.
In plain words
If you never received the goods, were charged without your consent or were charged twice for one purchase, you can ask your bank to dispute the payment. Under the payment system’s rules (Visa, Mastercard and others) the bank asks the merchant’s bank for an explanation and, if the claim is upheld, returns the money. The merchant loses the amount and usually pays extra fees, so a high share of chargebacks can lead to a higher tariff or termination of the contract with the acquirer. Your bank sets the deadlines and the evidence it needs.
Example. You paid for a ticket online, the event was cancelled and the seller will not refund. You file a dispute with your bank and attach proof (the cancellation email, screenshots of the correspondence), and the bank starts a chargeback.