This is a complicated question. The short answer is this: There are approximately 230 strings in a piano, each one under about 75kg of tension and wrapped around a pin that can be turned to tighten or loosen the string; the tighter the string, the higher the note. There is an ideal relationship between every possible combination of strings, and a trained listener can put all these notes at their best possible frequency. For complicated reasons these frequencies will not be exactly the same on any two pianos, so tuning a piano is like solving a puzzle, and no two pianos have exactly the same solution.
Also, tuning refers to the physical process of manipulating the pins so that the strings will be as stable as possible.