what does Voicing means?

In that sort of context it's the sorta
overall character of the amp/pickup/whatever in question.
 
i generally think that voicing means the frequency balance of and amp or pickup or whatever

not the envolope of the notes, so not the attack or the sustain etc..

for example mesa dual rectifiers have a different voicing to marshall JVMs. the mesas have a more lower mid emphasised voicing, wheras the marshalls have a higher mid emphasised voicing