Redeem travel vouchers or credits
Each airline has their own method for sending vouchers/credits.
Some airlines send you "electronic vouchers' via email that can be redeemed online or through a customer service agent for the airline.
There are also paper vouchers that are sent out via USPS that can be redeemed at a ticket office, through a customer service agent or at the airport.
Credits are often associated with a user's Frequent Flyer account and can be redeemed online.