How does sky propagation differ from line-of-sight propagation?

Electromagnetic waves - radio waves - propagate through the air and vacuum by altering the electro-magnetic (EM) properties of space. They will be reflected whenever the EM properties of the space change. Incoming radiation from the sun partly ionizes some of the air molecules in the upper atmosphere and, for some frequencies, this will reflect (or refract) the EM signal.

This allows the EM signal to be reflected over long distances - even around the world - multiple hops. This effect is only true for HF radio - frequencies up to 10MHz or so. And varies naturally, day and night.

A related phenomenon affecting the upper atmosphere, is that incoming meteorites may give temporary ionization tracks. These have been used for burst transmissions but are not suitable for broadcast communications.

Higher frequencies, 100 MHz or so, use line - of - sight transmission, for these high frequencies are not affected by the ionized layers.