This may be a bit late since Sam's post above.
Just in case this definition is still confusing to some I will try to give my interpretation of what it means. (but mostly because I had all this typed up)
I will admit to going out on a limb here so correct me if I am off base. (always willing to learn)
We can all relate to the extra-cool feeling when the evening air is damp and we probably have also heard the expression used by folks living in the desert states "....yeah, but it's a dry heat."
Moisture gives air the ability to hold more heat per given volume. 90F air with 90% relative humidity seems hotter than 90F air with 10% humidity. The same can be said for the cooling effect of 50F air with more or less relative humidity.
The virtual temperature calculation attempts to remove the moisture from the air and tell us what the air temperature would have to be for the "same feel" if it was at standard conditions.
To go a bit further....
The virtual temperature definition says it allows meteorologists to use the "equation of state for dry air". The equation of state takes into account a few laws of physics - notably Boyle's Law and Charles's Law. They simply state that (1) a volume of gas will be smaller if the pressure is increased and (2) a volume of gas will be smaller if the temperature is lower (assuming all other terms are the same).
The virtual temperature definition implies that in order to perform any type of calculation on the air, meteorologist must convert the temperature to standard conditions. In physics this is known as STP (or Standard Temperature and Pressure).
Now, does anyone know why meteorologists might need to perform these calculations? Perhaps to estimate the energy in a system?
Ok, not sure if I helped or not - but that is my attempt.