What are 'ghost flights', over which Qantas Airways will pay $66 million fine?

Australia's Qantas Airways agreed to pay $66 million penalty (around ₹550 crore) to settle a regulator lawsuit over the sale of thousands of tickets on "ghost flights". The term is used for long-cancelled flights, for which Qantas kept selling tickets to customers. Customers allegedly booked tickets on flights that were cancelled "two or more" days prior.