There are more than two hundred strings in a piano that are stretched at very high tension. When the tuning pins are turned, the tension can be tightened or loosened to alter the pitch. Tuning means adjusting the tension of each of the strings that the strings are in "harmony" with all the other strings. This makes the sound of the piano pleasing to the ears. If the piano is properly tuned, moderate playing will not have a big effect on the tuning. Pianos usually go out of tune due to changes in humidity.
This is the process of making fine adjustments to the piano action to compensate for changes due to wear, the compacting and setting of felt and changes in wooden parts. The changes can be a result of wear or of changes in relative humidity.