From promotional excerpts of AC6V's DXing 101: the Amateur Radio DX Reference Guide...

Suffice it to say that the geomagnetic activity, solar storms, X-Rays, flares, etc., can have an adverse effect on propagation. The Planetary A index relates to geomagnetic stability. Magnetometers around the world are used to generate a number called the Planetary K index. A one-point change in the K index is quite significant. K index readings below 3 generally mean good stable conditions, and above 3 can mean high absorption of radio waves. Each point change reflects a significant change in conditions. Generally the higher the latitude of the measuring station, the higher the K and A indices reported. This is because the effects of geomagnetic instability tend to concentrate toward the polar regions of the globe.

Oversimplification can be very misleading in the complex field of propagation, but in general for long distance HF, the rule of thumb is the higher the SFI (solar flux index) and the lower the A and K indices, the better the conditions on the higher frequencies. The A index should be under 14, and the solar activity low to moderate. If the A-index drops under 7 for a few days in a row and the SFI is up, watch for some really exciting intercontinental conditions.

A solar flare releases energy than can affect HF propagation: 1. ionizing radiation that arrives at earth immediatly; 2. a supersonic shockwave riding along the solar wind; 3. dense particles behind the shockwave that arrives two to three days after the flare.

Good DX contacts are possible immediately following a solar flare until sundown due to improved reflectivity and the higher MUF opening higher bands. Night time conditions on 80-40 can be excellent. About two days after a solar flare, the shockwave arrives on earth triggering a geomagnetic storm. ...

Classification of K-indices is as follows:

K0 = Inactive       K5 = Minor storm
K1 = Very quiet       K6 = Major storm
K2 = Quiet       K7 = Severe storm
K3 = Unsettled       K8 = Very severe storm
K4 = Active       K9 = Extremely severe storm

As with the K-index, the higher the A-index, the more unstable propagation becomes.

Classification of A-indices is as follows:

A0 - A7 = quiet
A8 - A15 = unsettled
A16 - A29 = active
A30 - A49 = minor storm
A50 - A99 = major storm
A100 - A400 = severe storm