Intervals are relationships between notes that have specific sounds. Each "note" is a specific frequency. The perfect intervals of any note are found by the relationships created by interaction of the two sound waves. As sound waves combine, the crests and troughs interact with each other either by canceling each other out or boosting one another.

There are infinite frequencies. However, it is impossible to create and instrument that can play infinite notes that are also distinguishable by the human ear. To solve this, the 12-tone equal temperament system, or 12TET, was created. This system divides an octave, or the interval between two of the same notes at double or half the frequency, into 12 equal parts. This sacrifices perfect in-tune-ness for practicality.

Before, for more accurate tuning, an instrument would have to be tuned to a specific key so the uneven, but more harmonious, intervals would sound much more stable. However, it was then impossible to leave that key. To play in another one, you would need another instrument tuned to that key. 12TET solves that issue by making the intervals equal and slightly out of tune so you could play is literally any of the 12 keys on the same instrument.

This concept can be used with any number of tones. In fact, lately, artists have been experimenting with other equal temperament systems as they feel the current one is oversaturated. However, the 12 tone one was simply the most popular because it was the one used when the Catholic church funded the creation of instruments for use in mass.