And for even more boring detail, HF does its thing by skipping the radio wave (signal) off the ionosphere (electrical/magnetic layer above the stratosphere) back down to the earth. Thus "Skip Distance". If the frequency is too high (MF/VHF/UHF etc) the signal will just go right through the ionosphere and not bounce back ("skip").
If the signal is powerful enough it can bounce off the earth and back up to the ionosphere for another skip, and if really powerful can bounce right round the world, letting you talk to yourself.
The ionospheric layer varies in height by day and night (breaking up at dawn and dusk, hence HF comms are not so good at those times). By choosing the right frequency you maximise the range (higher frequencies by day and lower at night).
You still have line-of-sight reception, like VHF, for the first hundred miles or so, but can have a dead band between the end of the line-of-sight signal and the first skip distance, and between the several skips. Once again changing the frequency can fill this gap (maybe).
HF antennas have to be variable in length or have adjustable gains to make them the right (electrical) length for the frequency chosen. See also "losing the fish".
Most ATC comms are done using Single Sideband (SSB) HF, which cuts out the energy needed to send both sidebands (Double Sideband as VHF/UHF uses) and put all the transmission energy into just one sideband, which has a logarithmic increase in transmitted energy and thus range. The upper and lower sidebands are identical in frequency and exactly opposite in amplitude, so one can be deleted without affecting the signal, provided both transmitter and receiver are using the same sideband. ATC has standardised on USB (Upper SideBand).
If frequencies became cluttered, messages could be sent at the same time over the same frequency on USB, LSB and DSB (upper, lower and double sideband) with no interference to the receiver. DSB is also called HF/AM transmission but is not much used any more, even by radio stations. Some Hams still use it and it can give surprisingly good results, especially at night when the ionosphere is low, giving a shallow angle to the skip signal, like skipping stones off water.
If you are trying to pick up a transmission on HF and it sounds garbled, try changing the selector to AM or to the other sideband if you have that choice, and it might fix the problem by matching the transmitter and receiver. If that does not work, tune off by a KHz or two.
And if you ever get a chance, listen to Speedbird 12 near Tiger intersection trying to contact "Bombay Bombay" (Mumbai Mumbai now). Some things never change.