Pitch raise/ Pitch drop: A piano has so many strings, that, if it hasn't been tuned recently, moved, or gone through a sudden change in the environment, may require a "Pitch Adjustment." The soundboard will shift and change as the strings are put into tension causing the rest of the piano that has been tuned to subsequently go out of tune. In this case a quick "Rough Pass" that gets all the strings in the ballpark of where they should be in order to equalize the tension to minimize shifting on the longer and more detailed "Fine Tune" pass.
Action: This is the whole mechanism between the keys and the hammers that allows the pianist to communicate effectively with the strings.
Regulation: Over time, the felt and wood inside the piano action will compress and warp. This changes the timing and spacing of the parts into positions that are less than optimal. For better control and communication between the pianist and the strings, these parts should be put back into place to facilitate a wider array of dynamics and a more apparent difference between smooth legato notes and short staccato notes.
Voicing: Much like how a guitar string sounds different depending on whether it is strummed with the pad of the finger versus the fingernail, a piano can have many tones or "Colors" as they are described by many, dependent upon the condition of the hammers, strings, and action. If there is one "Bright" Note or a soft "Dead" note, there are often changes that a technician can employ to make the voice of each hammer more similar to it's neighbors.
The Piano Technician's Guild also has some great resources and articles for the piano owner to read.
This is their selection of articles on piano care: https://www.ptg.org/piano/care
They also have some wisdom regarding buying a piano: https://www.ptg.org/piano/buying
For those who like history, here is a brief overview of the history of the piano: https://www.ptg.org/piano/history-pianos
