Ah, you've gotta love Harald. His idea of a simple explanation is a bunch of formulas! Well, he's German, and they just seem to be born with brilliant, analytical engineering-oriented minds.
The idea of harmonics is a way to analyse waveforms that aren't perfect sinewaves.
A sinewave is a pure tone, sounding like a whistle or a flute. Most sounds that we hear in the real world (e.g. in speech and from musical instruments) aren't sinewaves. Not even close. In fact they're a lot more complicated than even a repetitive waveform, because they include an "envelope" (attack, decay, sustain, release or more complicated than that) during which the timbre (as well as the volume) varies. But certain types of waveforms, sawtooth for example, have distinctive sounds.
The point of studying harmonics is that a simple, continuous waveform, like a triangle, sawtooth, or square wave, can be broken down into multiple sinewaves that are added together. Obviously, there is one sinewave at the fundamental frequency (the frequency of the more complex waveform), then other sinewaves at multiples (harmonics) of that frequency, and with different phase relationships, are added to that sinewave until you end up with the final waveshape.
Another way to look at this is that any regular waveform can be broken down into a sum of sinewaves at the fundamental frequency and multiples thereof, at different relative volume levels and phase relationships.
For example, if you use a filter, you can pick out the individual harmonics of a sound. Listen to the sound of a square wave, for example, through a tube or pipe with variable length. The tube or pipe acts as a filter, and resonates at frequencies that are related to its length. As you adjust the length of the pipe, it will pick out (resonate at) the various harmonics of the fundamental freqency, and you can actually hear them individually. This is a kind of evidence that those frequencies are actually present in the signal, although when you listen to it, it only appears to contain the fundamental frequency.
Harmonics in real world sounds occur because whatever is making the sound - a string, or a reed coupled to some air, or just some air in a pipe, or a resonating drum head - does not move with a perfectly sinusoidal (sinewave-shaped) motion, because of physical factors such as how it is mounted and supported, how the resonating chamber around it is formed, etc.
If every sound in the world was a perfect sinewave, every instrument, every human voice, and so on, would sound like a whistle or a flute. It's the timbre, i.e. the mix of harmonics, that gives sounds their identifiable character. You can tell whether a sound is a guitar or a saxophone or a kettle drum; this partly comes from their harmonics, but mostly from how those harmonics vary over time, during the course of the sound.
You can also distinguish simple continuous waveforms (triangle, sawtooth, square, sine) from each other, once you've heard them all, although they're much less "real-world"-sounding than the complex sounds produced by movement of air in the real world.
I'm not sure if this will help you or not. It's just a bit of a brain dump
Get some samples of real-world sounds and look at them with a waveform editor such as Audacity or Cool Edit Pro. Look at the waveshape, and how it changes over time, and listen to it carefully. Also, view sections of it in the frequency domain, using the FFT (fast Fourier transform) feature of the software. Try to correlate what you hear with what you see.