HF signals need to bounce off the ionosphere to travel any distance. The more ionized the ionosphere is, the more effectively hf radio waves refract off it. High solar activity leads to higher ionization.
In ham terms, this is know as “band openings”, eg when ionospheric conditions allow hf at a given frequency to refract off the ionosphere.
Beyond the horizon transmission (radio propagation) is by bouncing signal off (refracting) the Ionosphere, which is more effective when Solar activity is at its maximum. At Solar minimum, about fifteen to 30 megahertz is screwed.
Low solar activity means that there is less EUV (extreme ultraviolet) radiation hitting the ionosphere. This leads to weaker plasma formation, as well as shifting geometries of the plasma layer. In general the lower and weaker plasma tends to restrict the range of HF transmissions attempting long distance communications via refraction through ionospheric plasma.
