Variable wattage is wonderful.....been using it for almost two years.....I no longer own Variable voltage devices.....
To answer your question, What does it mean? :
Variable voltage (voltage regulation) maintains the voltage to the atomizer/carto, etc... as you change atty/carto,etc to one with a different ohm rating, you just adjust the voltage again to compensate for the different ohm rating.
Variable wattage (power regulation) maintains the wattage to the atomizer/carto, etc....as you change atty/carto, etc. to one with a different ohm rating, you don't have to adjust the wattage, as it automatically compensates for the ohm difference to maintain the wattage...
How do you set it? :
Using either Variable Voltage or Variable wattage you basically adjust it to the voltage/wattage setting until you find your best vape experience.
If you change the atty/carto to a different one with a different ohm rating you need to re-adjust the voltage setting (when using variable voltage) again to get your needed vape experience, and you need to do nothing with variable wattage (when using variable wattage).
That is the short answer to your question....