What is tuning?

"Tuning is the act of tightening or loosening the strings to adjust the frequency at which they vibrate, i.e. their pitch. A piano is made up of more than 220 strings, each bearing tension of between 75 and 85 kilos depending on the model. Each note in the mid and upper range has three strings which must be tuned to the same frequency: the unison. The usual reference point is A4, 440Hz, which is 440 vibrations per second. Tuning is generally done according to the principle of equal temperament. The reason why tuning a piano is difficult is that the tuning pin that holds the string must be wedged into the wooden pinblock so that the tension, and therefore the frequency, remains constant. This has to remain the case for an extended period of time:
if the tuning pin cannot be held in place, the piano will not stay in tune for long."