Tuning is the process by which the piano is brought into systematic regulation with itself. it includes tensioning /re-tensioning of over 200 piano strings to the same tension level
The process begins by setting the instrument to the desired pitch to which it is to be tuned. Although there is a standard pitch that has been agreed by all musicians & musical instruments manufactures, which is @A440. This a vibration of note A above middle C which vibrates at a cycle of 440 per a second(and this is the international pitch standard)
By always maintaining your piano at standard pitch, you create long-term tuning stability because the strings and structure stay in equilibrium. You also ensure proper ear training because you always hear your music in the correct key.
The tuner starts this by setting the "temperament." This is the 12-note pattern that governs the entire tuning of the piano. It is based on a mathematical formula that dictates that all the notes in this 12 note pattern be spaced equally apart in distance from each other i.e. the 3rds, 4ths, 5ths, etc..
This "scale" has been worked out by musicians down the years, and become to be known as "Equal temperament."