Tuning is the most common service that pianos receive. It is a procedure of changing the tension of the piano strings so that they are in pitch to a given reference frequency (usually setting A4, the fifth A from the bottom of an 88 note keyboard to the standard concert pitch of 440 Hz, or cycles per second), and adjusting the remaining notes on the piano to conform to a harmonically pleasing twelve tone scale of half-steps.
This is a complicated description for a complicated process of setting a piano so that it sounds musically pleasing when pieces are performed in different keys. It is not as simple as setting each note (of which many have three strings per note) to its theoretical Hertz frequency, as this does not make the piano sound "in tune" with itself; music sounds harsh and dissonant. Pianos must be tuned relative to themselves, that is, so that intervals such as octaves, fifths and fourths sound tuned despite the fact that they may be different than mathematical scale frequencies.
Pianos need tuning; they were designed for it. Once a year is the minimum that a piano should be tuned; twice a year is ideal. A regular tuning schedule will help the technician keep the piano sounding (see voicing) and playing (see regulation) in peak condition. Without regular tuning, pianos drop in pitch and require greater effort to keep them on pitch. Pianos that "never need tuning" in fact are never tuned, as all pianos require regular tuning. It is hard to judge "good" tuning in a piano if a person has become used to an out of tune instrument.