A spark that jumped
Hertz built a transmitter with a spark gap, and a little way off a simple receiver, a wire loop with a small gap. When a spark jumped in the transmitter, a tiny spark jumped in the loop too, even though nothing connected them. He measured the wavelength and showed that the waves reflected and refracted much like light.
"It's of no practical use"
When students asked him what his waves would be good for, he is said to have replied: nothing, they merely confirm Maxwell's theory. A few years later they were used to build the Radio, and today they carry every mobile phone call and every Wi-Fi router's signal.
A trail towards photons
During his experiments in 1887 he noticed that ultraviolet light helped a spark jump. This was the photoelectric effect, which Albert Einstein would explain 18 years later.
Hertz in every device
A frequency of 2.4 GHz means 2.4 billion oscillations per second. The same unit is used for the clock speed of a processor and for a monitor's refresh rate, for example 144 Hz.