The most important aspect of regular piano maintenance is tuning. Your piano should be tuned at least twice a year to maintain a clear crisp sound at the correct pitch. I practice the centuries old aural (by ear) tuning method, often tuning the entire piano several times  to ensure a stable tuning at the proper pitch. Included in the Piano Tuning/Service appointment are: Tuning the piano to Concert pitch; Minor repairs and adjustments such as sticking ,clicking, sqeaking or sluggish keys or pedals; and cleaning the keytops. It is essentially whatever your piano needs that can be done in a couple hours.  If your piano has not been tuned in several years, it will require more than one tuning appointment to hold a fine tuning at standard pitch.