Harmonics are frequencies that are integer multiples of some fundamental frequency in the circuit. If your system has a signal that is not a pure sinewave (single freq), then harmonics will exist.
If your system has a squarewave signal operating at 100 kHz, then you'll have a fundamental signal at 100 kHz and harmonics at 300 kHz, 500 kHz, 700 Hz, etc (since squarewaves contain only odd harmonics). Other waveforms will also have even harmonic components. So, if it's not a pure squarewave, you'll also see harmonic signals at 200 kHz, 400 kHz, 600 kHz, etc.