What is it ?
A harmonic sound is a sound that has a frequency that is an integer multiple of the fundamental frequency of a sound. For example, if the fundamental frequency of a sound is 100 Hz, then frequencies of 200 Hz, 300 Hz, 400 Hz, etc. would be harmonics of this sound.
Harmonic sounds are often produced by sound sources that vibrate regularly, such as the strings of a violin or the vocal cords of a singer. They are also produced by synthetic sound sources, such as electronic keyboards and sound synthesis software.
It is relatively easy to recognize a harmonic sound. They generally have a clearer, more crystalline sound quality than non-harmonic sounds, and they can often be perceived as being more "pure" or "pleasant" to the ear.