Piano tuning is the act of making minute adjustments to the tensions of the strings of an acoustic piano to properly align the intervals between their tones so that the instrument is in tune. The meaning of the term ‘in tune’, in the context of piano tuning, is not simply a particular fixed set of pitches.
To tune a piano you need to understand the basic physics of Sound generation, the Dynamics, harmonic structure, trichords, beats & sound cancellation. When playing the piano you see your white keys and black keys. Not knowing that some keys can have up to 3 strings attached to it.

Tuning a piano is adjusting the tightness of each individual sting at a time. And repeat this over 200 times per piano.