The piano is quite an amazing instrument like no other. Usually containing 12,000+ parts and maintaining pressure on the strings equivalent to the weight of many cars, most modern consoles have somewhere in the neighborhood of 37,000-38,000 pounds of pressure on the strings. These are bowed across the soundboard and the cast-iron plate.A 5’8”-6′ grand can have as much as 40,000 lbs and it only increases as the piano gets bigger.
Every time there is a temperature change the metal pieces in the piano expand and contract. Every time there is a humidity change all the wood pieces expand and contract. This puts the piano in a constant change of flux. Factor in extraneous factors (ie;moves), and you can see why the instrument goes out of tune.
Tuning is the process of adjustment of pressure on the strings so that each note is approximately the same distance apart. Most pianos that have a consistent tuning history will be ok if they are tuned once a year. To be really good they should be tuned at least twice a year. Learn more as to why pianos go out of tune.