Let's suppose that a cantilever is vibrating at a frequency of 2Hz. That means that it goes up and it comes down twice per second. I don't understand what the term harmonic means. I can't grasp how the vibrating rod moves twice and, at the same time, four times or eight times per second.
Descriptions of harmonics in engineering texts that I study explain nothing. These explanations suggest " an event that happens twice, four times, etc at the period of a vibration".
Any comment, suggested text or answer will be useful.