A chargeback happens as the result of a donor contacting their credit card company to dispute the transaction. Their bank or card issuer will immediately remove the funds from your payment account and set them aside while they conduct their review of the dispute.  

This usually happens when a donor doesn't recognize the charge on their account and calls their card company out of concern. If you know the donor personally, try reaching out to them to remind them what their donation was for, and they can call the card provider to drop the dispute,  or donate to your campaign again.

Otherwise, you can concede to the chargeback and return your donor's payment back to them.