Amateur Radio is actually Experimental Radio. Certain frequencies have been set aside for use by experimenters, and "we" consider that the amateur radio frequencies are the "National Parks of the Airways." We Amateurs are a bunch of enthusiasts who push the boundaries of radio. Many amateurs have found uses for radio that would never have been found otherwise. Not all discoveries are commercially viable, but amateur radio operators enjoy pushing the limits and finding new ways to propagate signals around the planet. Most amateurs are very observant, and can quickly realize when something is not "normal." They often write up and share their experiences to see if anyone else has had a similar experience, then discussions follow. A recent example of this is what several of our club members are doing and that is to send microwave signals across the ocean at just above the water line. The original thinking was that microwave signals would not propagate just over the surface of the ocean, that they had to be far higher, as in several hundred metres, but a couple of members had experiences where they had better contacts on windy days over a salt water path. This was against the current thinking. So they continued to experiment, and the experiments are still continuing. No conclusion has been reached, but it might be that the radio waves are propagating through the salt air just above the water line.
Lets start with frequency and wavelength.
Frequency is rated in Hertz, after Heinrich Hertz, the German scientist who proved some previous mathematical beliefs about electromagnetism. It was then discovered that there was also an electrostatic field as well as the electromagnetic field, and that they are at right angles to each other. A Hertz is one full cycle per second, that is to say, that the voltage would go positive then negative, and repeat for ever. If the frequency is high enough it can "propagate", that is to radiate away from the antenna and be collected by another antenna some distance away.