Plain English Meaning
Anything you're doing or talking about that isn't needed to fly the airplane safely right now.
Full Definition
Under the Sterile Flight Deck Rule, any conversation or task that is not directly required for the safe operation of the aircraft during critical phases of flight (typically taxi, takeoff, landing, and all flight operations below 10,000 feet, excluding cruise). Examples include casual conversation, personal calls, sightseeing commentary, eating meals, and discussions of non-operational matters.
Context Anchor
Used in the sterile flight deck rule, especially during taxi, takeoff, landing, and other times when the crew must keep attention on operating the aircraft.