The pitch of a piano refers to its relative sharpness or flatness. In technical terms, the pitch of any given note can be measured in hertz, which is vibrations per second. The very lowest note on (the left hand side of) a piano usually vibrates around 27 1/2 times per second while the highest (on the right side) exceeds 4100 times per second.
Standard pitch was adopted around 1925 when all musicians agreed that the "A" above middle "C" should vibrate at 440 times per second. When a piano is tuned to standard or concert pitch, it is said to be tuned to A440. That note is usually set to a tuning fork or an electronic device and the rest of the piano tuning is based upon that note.
Click here to hear A440 and compare your piano.
Seasonal climates changes will affect the pitch of a piano as well as the tuning.
Why should your piano be tuned to standard pitch?
Actually it is up to you if this matters to you - the piano doesn't really care and it will sound fine if it is tuned where it stands (which may be at standard pitch anyway). Here are reasons why you would want it tuned to standard pitch:
It should be pointed out that manufacturers generally design their pianos to be tuned at standard pitch for the best tone. Also: Having a pitch correction performed is a waste of money if you are not committed to tuning the piano at least twice a year because the pitch is likely to drift again if allowed to move along unchecked by regular tuning.
Some older pianos with tuning stability issues may not be able to hold a tune at standard pitch and may need to be tuned somewhat flat from A440.