VFR stands for Visual Flight Rules; these are set of rules under which pilots must operate an aircraft in weather conditions clear enough to allow the pilots flying the aircraft to see where the aircraft is going.
The weather simply decides on the decision to fly. VFR requires that the pilot is able to see outside the cockpit, to be able to control the altitude of the aircraft and avoid obstacles and other aircraft.
VFR flight is not allowed in class A airspace regardless of the meteorological conditions. In some countries, VFR flights are permitted at night.