The action is the keyboard unit of the piano. When played, many parts of the action wear out slightly. When that wear gets too great, the action gets out of balance. The way to know if a piano needs regulation is to see how hard or easy it is to play. When an action is in good regulation, it is easy to play very soft as well as play very loud. The farther out of regulation it is, the less expressive it is, that is less loud and less soft. Yamaha, Kawai, and Steinway recommend a full action regulation every 8-12 years. This ensures that your piano is at it's optimal playability all the time. A typical upright action regulation can be done in your home in 3-4 hours. A typical grand action regulation is done in my shop (12-15 hours) and in your home(1-3 hours) and takes approximately 2-3 days.