Plain English Meaning
The fastest you are allowed to fly while the flaps are lowered. Go faster than this with flaps down and you risk bending or breaking them.
Full Definition
The highest speed at which an aircraft may be flown with the wing flaps extended to any approved setting. Exceeding this speed with flaps down can damage the flap structure or the mechanism that holds them in position. On the airspeed indicator, this limit is marked by the upper end of the white arc and is also published in the Pilot's Operating Handbook as V_FE (velocity, flaps extended).
Context Anchor
Seen in airspeed indicator color-code discussions, airplane operating handbooks, and flight manuals when checking flap .