To regulate a piano is to adjust all of the parts so that they operate uniformly and efficiently.
Pianos are full of wood, felt, and leather parts that do eventually become worn to the point of needing replacement.
This adjustment is done on the mechanism of the piano, to ensure the even performance of each key. Which usually takes several hours of work.
After all the worn and broken parts have been replaced, the process of making adjustments can begin. Some of the adjustments that are made during regulation include shaping the hammers to eliminate the grooves and restore a proper strike point, key leveling, string leveling, mating the hammers to the strings, setting the distance from the strings to the hammers, adjusting how much the keys “dip” when depressed, adjusting the dampers to all lift the proper amount and simultaneously.