Voiding an invoice reverses every product line: each item that was sold goes back into stock, and any serial numbers come off the invoice and back into available serial inventory.
If the invoice had been paid, the payment is also reversed and shows as a refund. If labor time was clocked, it's left intact - voiding doesn't un-clock work that already happened.
A voided invoice is kept in the system as a record (status: Voided) - it doesn't disappear. The Drawer Summary and Daily Sales reports exclude voided invoices from their totals.