Or....
It is the airspeed at which the machine uses minimum power to remain in flight.
As airspeed increases the blades get more efficient and so less power is required to sustain flight. The blades continue to get better as airspeed increases, but drag on the blades and the body of the helicopter also increase as you gain speed and these require power to overcome.
At lower speeds the improvement in blade efficiency (less power needed) outweighs the combined increase in drag (more power needed) and so the total power required to keep the helicopter in the air falls as speed increases. However, the rate of increase in drag gets higher as the speed gets higher, so there comes a speed when either increasing speed (increasing drag more than blade efficiency) or reducing speed (reducing blade efficiency more than reducing drag) will mean that the total power required to stay in the air will increase.
Simples!
The technical explanation you have above, or is available from any Principles of Flight training manual for PPL(H). The curve for any particular helicopter can be found in the POH.
John