Tuning is adjusting of strings on the piano, so they sound at correct pitch. The standard is A440 Hertz. A piano tuner makes fine adjustments  to the tension of the strings so that they sound at the correct pitch. When the piano is in tune, it has a clearer, more harmonious sound and provides a more enjoyable experience for the beginner student and accomplished pianist alike.

FYI: it is recommended by professional piano technicians at Classic Piano Centre to allow 4-6 weeks settling time after piano has been moved to play each and every key, and later to perform tuning after delivery.