Tuning involves tightening and loosening the tension of the steel strings in a piano, so that they are organized to sound beautiful, or in harmony, together. The higher the tension of a string, the higher the pitch of the sound, and conversely, the looser the string the lower the note of that string will be.
In most pianos there are more than 230 strings and all must be properly tuned to each other to sound musical.
The primary reason piano go out of tune is due to fluctuations in the amount of moisture in the air; in other words - changes in humidity.
Most piano manufacturers recommend at least four tunings in the first year and two tunings per year thereafter for a typical home piano. This is because of the changes in humidity, and hence tuning due to the changes in season.
Some people like to go as long as a year between tunings. This should be considered minimal and certainly should not go any longer than that. On the other hand, some people who have more sensitive ears, and consider themselves artists, have their piano tuned four times a year or more. Concert halls and recording studios usually tune every performance.