Voicing involves making the tone of the entire piano brighter or more mellow. Making it brighter is done, either by ironing or fine sanding the hammers, or by applying a chemical felt hardener. Making it more mellow is usually done by sanding or steaming the hammers, or pricking the hammers with needles, which softens the felt.
Sometimes some individual notes stick out as too bright, or not bright enough compared to the rest. In this case some fine voicing is required, which involves sanding and needling, to even out the tone so that it is consistent throughout the piano.