Have you ever been at the airport and heard a gate agent asking for volunteers to be bumped? The plane is oversold, or has a weight limit, and the agent is offering hundreds of dollars to anyone who will volunteer to take a later flight.
What you want is to volunteer to be bumped before others have time to decide if they can postpone their travel. This is a decision that should be made before the announcement, not afterwards.
Some airlines usually offer $300, $400 or more of flight credit per bump, others usually offer a travel credit good for round-trip to any domestic (non-Hawaii) destination served by the airline. In either case, you normally have 12 months to use or lose the benefit.
Occasionally you may be bumped from coach into First or Business class on your new flight. It’s worth asking for this benefit.
Bumps are excellent ways how to fly free.