Tuning a piano is adjusting the tension of the piano's more than two hundred strings with a specialized tool called a tuning hammer. Because most of a piano is made of wood and wood swells and shrinks with changing humidity, the tension of the strings correspondingly change. This changes the pitch of the notes, and since the string tension does not change uniformly throughout the piano, it will begin to sound "out of tune." Installing a humidity control system can minimize these changes, requiring less frequent tunings.
Piano manufacturers recommend that your piano get tuned at least twice a year. You must decide how frequently to schedule tunings for yourself based on what your own needs are. Keep in mind that pianos that have not been tuned for several years and have gone quite flat will need an additional service called a pitch raise. Also, regular tunings will mean that the technician can spot possible regulation or repair problems earlier, often saving you from large, costly repairs later. For every tuning service call I provide a complete inspection of the piano. I also adjust the pedals and tighten bench bolts, pinblock bolts, and plate screws.