IonoFlux field guide
Sunspot number
Sunspots explained for beginners: why active regions can influence HF radio.
In simple terms
Sunspots are darker, magnetically active areas on the Sun. More sunspots often accompany more ultraviolet radiation, which can make the ionosphere more useful for HF.
How to read it
The number changes slowly compared with a flare or storm, so it is best for understanding the general phase of the solar cycle. It does not tell you whether a specific contact will work today.
A practical example
A rising sunspot number can make 15 m and 10 m more interesting over time, but a sudden geomagnetic storm can still make those bands poor for a few hours.