Plain English Meaning
It means that whenever control of the aircraft is being passed from one pilot to the other, both pilots say it out loud and confirm it, so there is never any doubt about who is actually flying.
Full Definition
A flight crew procedure in which the transfer of aircraft control between pilots is confirmed verbally so that, at every moment, both pilots know exactly who is flying the aircraft. The standard exchange uses three statements: the transferring pilot says 'You have the flight controls,' the receiving pilot replies 'I have the flight controls,' and the transferring pilot confirms 'You have the flight controls.' Control is not considered transferred until this exchange is complete.
Context Anchor
Used in flight instruction when the instructor and learner are practicing maneuvers, correcting mistakes, or transferring control of the aircraft.