Plain English Meaning
The flaps are pulled all the way back into the wing, so the wing is smooth and clean with nothing hanging down.
Full Definition
The configuration in which the wing flaps are fully stowed against the trailing edge of the wing, presenting a clean wing with no flap deflection. In this state the airplane has its lowest drag and highest stall speed for a given weight, and is the normal configuration for cruise flight.
Context Anchor
Seen in descent, approach, climb, and checklist discussions when describing how the airplane is set up for .