What is a flight change fee? But what exactly does that mean?

What is a flight change fee?
But what exactly does that mean? Change Fee: This is a fee the airline charges to make a change to a reservation, typically to rebook on a different flight. Cancellation (Refund) Fee: This is a fee the airline charges to cancel a reservation and process a refund.

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What is a flight change fee? But what exactly does that mean?