What does ticking the "Void Transaction" checkbox do when processing a refund?
Void is a special attribute of refunds. Some payment gateways do not allow you to refund before the funds are settled. Settlement often happens within 24 hours. Voiding the transaction allows you to cancel the transaction before it is settled.
Most transactions will not require ticking this checkbox, and not all payment gateways require this step. If you are unsure, leave it unchecked.