Final answer:
In chapters 19-21 of Treasure Island, when the pirates wave a white cloth, it signifies a truce and a desire to negotiate peacefully.
Step-by-step explanation:
In chapters 19-21 of Treasure Island, when the pirates wave a white cloth, it signifies that they want to negotiate and are sending an unarmed man. This action is a gesture of truce and a request for parley or discussion with the other party. It is a way for the pirates to communicate their willingness to negotiate peacefully rather than resorting to violence.