A "void" will reverse a "purchase", "capture", or "pre-authorization" and release the funds back to the cardholder. It can only be performed to the point of settlement.
IMPORTANT: Void vs. Refund
If the original transaction was processed today and has yet to be settled, you should perform a "void" instead of a refund. A void will avoid any transaction fees and the original transaction will not appear on the cardholder statement. However, if a transaction has already been settled you will need to perform a Card Refund instead.