A concentration of sound wave energy that relates to a fundamental frequency.
A Sine Wave is a simple sound and a concentration of soundwave energy at a single frequency.
Harmonic sounds are made up of a number of sine wave type components, making a complex sound. If the individual sine wave components within a single sound are related to one another by simple mathematics (whole number multiples of a fundamental frequency (f), for example: f2, f3, f4, f5,) they can be described as having a harmonic relationship. The root of this harmonic relationship is called the fundamental frequency.
In this spectrogram you can see the many harmonics that make up this sound.