The sound the piano makes is made up of a lot of features, but the two most important ones that can be readily manipulated by the piano technician (and should be) are tuning and voicing.
We've adequately described tuning above. But the second half (or first half) of tuning is "voicing."
Voicing is the manipulation of the hammer head felt so that when it strikes the string it brings forth the most beautiful tone possible. A well voiced hammer produces a sweet, clean, velvety tone (timbre) of proper loudness directly proportional to the effort exerted by the performer. A poorly voiced hammer (one that is too hard or soft, or is a mixture of hard and soft spots) will produce a mushy, dull or bright, harsh howling tone (timbre) and does not feel to the performer to match his effort for loudness. The timbre (tone color, flavor, sensation) is either good, bad, or somewhere in between (and so is the loudness).
What a technician does when he/she voices the piano is to soften or harden the hammer felts, blending them evenly through the entire range (all 88 hammers) so that the tone is even and smooth throughout the entire piano. Think of it this way. Generally speaking, when we change notes, we only want to hear the pitch change from note to note. We do not want to hear the tone change. In worst case scenarios, you will play a piano that has serious voicing problems and as you run up or down a scale, it will suddenly sound like you changed pianos right in the middle of the scale--not only are the pitches changed but suddenly the tone or volume also changed. Sometimes if sounds like (and feels like) one note "jumps" out from the rest and "bites" you every time you play that note. Do you find yourself not liking certain notes on your piano? Voicing problem! Do you find yourself either trying to play certain notes softer than others or harder than others? Voicing problem! (and/or regulating problem discussed below). Do some notes sound real pleasant to you and others sound harsh or biting? Voicing problem! You get the point; a fine tuning on a poorly voiced piano will still not give satisfying results, and still be far from what that piano is ultimately capable of.
Generally speaking, hammer felts harden over time because of being "hammered" into the strings over and over. The felt gets compacted, as well as groves getting cut into the felt. If you look at a hammer you will see it is actually a wooden wedge covered with felt. Image the sound that would be made if all the felt was removed and the string was getting struck with just the wooden wedge--a terrible, minimal crash, clunk, ?@%# kind of sound. Now picture the fact that over time, the felt is getting harder and harder. Thus the sound (tone) being produced is moving more and more toward that which would be made by the wooden wedge by itself. Voicing restores the felt characteristics, thereby regaining the optimal tone on each and every note.