The piano action consists of all the parts between the piano key, and the piano string it strikes to produce a tone.
The action consists mainly of wooden parts with many leather and felt pieces. Over many years of playing and age the leather turns to powder, the felt compresses and wood begins to dry out or warp, thus making the piano play improperly. This is where action regulation comes into play. The action can be regulated throughout the years to accommodated this wear. Until eventually the felts, leathers and hammers must be replaced. Once replaced and properly regulated these piano actions will last for many more years to come.
There are many levels of action restoration depending on the age of the piano, the condition and the environment the piano has been in for the past 80 years. keep in mind every time a piano key is played parts are being compressed and wear is occurring.