The pianos sound is produced by hammers hitting strings. A piano hammer is composed of felt, tightly compressed around a wooden core and the type and density of the felt has a drastic affect over the timbre of the piano. Voicing is the task of the technician to delicately needle the hammer felt in order to shape the timbre of the piano.
If you find your pianos tone uneven or brighter/harsher than you would wish, look no further, voicing may be the answer you've been looking for.