Regulation is the adjustment of the mechanical aspects of the piano to compensate for the effects of wear, the compacting and settling of cloth, felt, and buckskin, as well as dimensional changes in wood and wool parts due to changes in humidity.
The parts of the piano that are regulated are the pedals and trapwork system, the damper system, and the action. The action is the mechanical part of the piano that transfers the motion of the fingers on the keys to the hammers that strike the strings. It is comprised of over 9,000 parts which require adjustment to critical tolerances to be able to respond to a pianist's every command. The trapwork is the assemblage of levers, dowels and springs that connects the pedals to the action effecting sustain and dynamics. The damper system is the mechanical part of the piano that stops the vibration of the string when you release the key and is controlled by the key and pedal systems.
While tuning corrects the pitch of your piano - it is only one component of a complete maintenance program. Regulation attends to the touch and uniform responsiveness of your action, all vital to making each performance pleasurable. In addition, regulation ensures that your instrument is capable of producing a wide dynamic range. a critical factor, particularly in pianissimo passages. If you instrument displays a lack of sensitivity or a decreased dynamic ranges, it's a candidate for regulation. If you notice that the keys are not level (some higher or lower than the rest), the touch is uneven or that the keys are sticking, the need for regulation is indicated. However, a sluggish action or deep grooves in the hammers indicate the need for reconditioning or repair.
Your piano should be thoroughly cleaned, the action regulated, and the hammers reshaped and voiced approximately every three years, or as needed, depending upon the usage and quality of your instrument.