Plain English Meaning
The fastest speed you should normally fly at. You can go faster, but only if the air is smooth, because rough air at higher speeds could damage the airplane.
Full Definition
The maximum speed at which an airplane may be operated in smooth air. Above this speed, the airplane must only be flown in smooth air, and even then with caution, because turbulence or abrupt control inputs could cause structural damage. On the airspeed indicator, VNO is marked by the upper end of the green arc and the lower end of the yellow (caution) arc.
Context Anchor
Seen on the airspeed indicator and in the airplane’s operating limitations, especially when checking speed ranges for descent or cruise.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed. The 'NO' stands for ' operation.' So VNO literally means 'speed, normal operation' -- the top of the .