The primary source of musical sound in a piano are the vibrations made by the strings. Those strings are caused to vibrate by hammers striking them. The qualities and dimensions of the felt on the hammers plays a significant role in the tone produced when they strike the string. Piano voicing involves working with the felt on the hammers to produce a desired tone.
Environmental factors and/or considerable use over time can affect the hammer felt and create conditions where the tone is considered either too shrill and bright, or too mellow and dull. Sometimes a pianist just has a particular preference and wants a piano technician to adjust their piano’s tone.
Voicing could include working with just a few hammers to equalize the tone and make them more consistent with their neighbors, or voicing all the hammers in the piano.