WHAT IS TUNING?
Tuning is the adjustment made to the tuning pins by the piano tuner to bring each string to its proper tension. There are approximately 230 separate strings on the piano, each with a total of between 160 and 200 lbs. of tension.
The standard international pitch is 440 cycles (or vibrations) per second. This is the pitch that sounds when “A” above middle “C” is played thus we call it A440.
If a piano is allowed to go 1/2 step below pitch, it can mean a difference of 3000 to 5000 lbs. of tension on the strings and plate. When your tuner tells you your piano needs a pitch raise, he means the tension needs to be increased so he can bring the sound back to standard pitch (A440).
HOW OFTEN SHOULD I TUNE MY PIANO
All pianos go out of tune whether they're played or not because of expansion and contraction of the wood due to atmospheric changes. A good rule of thumb for a piano in a home near my location would be twice a year – when the heat is turned on for the winter and then again when turned off for the spring.
Really the best answer is “As often as the user feels it is necessary” and pianos used in concerts, recording studios, TV and radio are tuned before each performance.