'Tis what it says! Variable Voltage OR Variable Wattage modes.
Variable Voltage = In this mode, you can adjust from 3.0 to 6.0 Volts in 0.1 volt increments. The voltage that you set will be maintained as accurately as possible by automatic adjustment of current delivery, as the battery drains.
Variable Wattage = In this mode, you can adjust from 3.0 to 15 Watts in 0.5 Watt increments. The wattage that you set will be maintained as accurately as possible via analysis of voltage and current delivery, as the battery drains.
Both could really be considered as "Constant Voltage" or "Constant Wattage" rather than variable, as neither VV or VW vary by themselves but only via user input of the + and - buttons.
Most folks forget the VV setting after a bit, because, as said above, the VW setting, by varying the voltage to suit the watts, all by itself, is just so much easier, no thinking involved.