Plain English Meaning
A short spoken exchange between two pilots that makes it absolutely clear who is flying the plane right now, so neither person assumes the other is doing it.
Full Definition
A formal verbal procedure used to transfer control of an aircraft between two pilots, ensuring that at every moment one pilot — and only one pilot — is clearly responsible for flying the airplane. The standard three-step exchange is: the pilot handing over control says 'You have the flight controls,' the pilot taking control responds 'I have the flight controls,' and the original pilot confirms with 'You have the flight controls,' often accompanied by a visual check that the other pilot's hands are on the controls.