Voicing is one of the true art forms in the world of piano technology. There are lots of tuners that do a good job and most of them can regulate a piano somewhat, but voicing is what separates the good technicians from the great ones. Voicing is the ability to change the sound of the piano or the tone of the notes it makes. You could draw an analogy between adjusting the bass and treble on your stereo to voicing, but where those knobs adjust the sound for the entire system, voicing is done on a note by note basis.
Voicing involves several methods of manipulating the strings and hammers in the piano. The hammers of a piano are made of highly compressed wool felt. Watch the video below to see how hammers are made but understand that they're created under huge amounts of pressure. This makes the hammers very hard but not of the same hardness all the way through. Voicing can involve any or a combination of the following:
Voicing is a long process where each note is evaluated by playing it and comparing it to the surrounding notes as well as the whole piano. Different techniques are used to move the sound towards the customer's desire and then the notes are checked again. This process repeats over and over again until the piano sounds acceptable. A well voiced piano may be bright, crisp and bell-like or rich, warm and mellow. The key is consistency across the entire range of the piano.