A great article on the Introduction to HF Propagation. Here is it as well.

A guide to understanding Solar Indices
 
SSN: Smoothed Sunspot Number 
LF: Low Frequency (30 - 300 kHz)
MF: Medium Frequency (
300kHz-3000 kHz)
HF: High Frequency ((HF: 3MHz-30MHz)
VHF: Very High Frequency (30MHz-300MHz)
SF: Solar Flux
A Index: A daily index of geomagnetic activity derived as the average of
          the eight 3-hourly a indices.
K Index: A 3-hourly quasi-logarithmic local index of geomagnetic activity
          relative to an assumed quiet-day curve for the recording site.
          Range is from 0 to 9.  The K index measures the deviation of the
          most disturbed horizontal component.

The propagation of radio signals in the medium frequency band (MF: 300kHz-3MHz), the high frequency band (HF: 3MHz-30MHz) and even the very high frequency band (VHF: 30MHz-300MHz) is largely influenced by the condition of our sun and, through the emissions of our sun, also the earth magnetic field that captures these.

 A Index and K Index  (How to read them)

The Earth's magnetic field is continuously monitored by a network of magnetometers. These readings are converted into the A and K index values.

WHAT ARE THE A AND K INDEX?

The explanation of the solar cycle is based on ‘smoothed’ figures, that is the average figures for each month are used rather than using daily figures. The reason for this is to give a more accurate picture than using daily figures would. Day to day solar activity can vary dramatically, so using these figures for propagation prediction can lead to unsatisfactory results. As well as variations in the geomagnetic flux, there are two other measurements of the magnetic field activity (caused by interactions of the solar wind the magnetosphere and the ionosphere) that are used in predictions. These are the A AND K INDICES. The K index is a 3 hourly reading of the magnetic activity compared to a standard ‘quiet’ day taken at various observatories. This index was first introduced by J. Bartels back in 1938 and consists of a single number in the range 0-9. The planetary K index (Kp) is the mean or average index of readings from 13 observatories located between 44 and 60 degrees latitude each side of the equator. The planetary index measures solar particle radiation by it’s magnetic effects. The A index relates to geomagnetic stability and is derived from the K indices of the previous day. The Planetary A index (Ap), is derived from the A indices of the world wide network of magnetometers.
 

K INDEX:

The K index is computed once every three hours (eight times a day) and the values can range from 0 to 9, with 0 being inactive, and 9 representing an extreme severe storm condition. The values are quadi-logarithmic.
  • K = 0 Inactive
  • K = 1 Very quiet
  • K = 2 Quiet
  • K = 3 Unsettled
  • K = 4 Active
  • K = 5 Minor storm
  • K = 6 Major storm
  • K = 7 Severe storm
  • K = 8 Very severe storm
  • K = 9 Extremely severe storm

A INDEX:

The A index is linear, and is computed from the eight previous K index values. It ranges from 0 (quiet) to 400 (severe storm).

  • A = 0 - 7 Quiet
  • A= 8 - 15 Unsettled
  • A = 16 - 29 Active
  • A = 30 - 49 Minor storm
  • A = 50 - 99 Major storm
  • A = 100 - 400 Severe storm

Generally, propagation conditions are best when the A index is 15 or lower, and the K index is 3 or lower.

When looking at the A or K figures, the lower the better as far as propagation conditions go! For best conditions look for A and K indices of 0 which have been sustained for a few days, coupled to a high SFI (above 250). You can then expect excellent propagation on the HF bands (unless one of the short lived solar events described below occurs at the same time then, of course, that theory doesn’t work!). K indices of 3 or below are considered to be ‘quiet’, whereas a k index of 9 indicates severe geomagnetic storm level. For the Lower bands (1.8, 3.5 and 7MHz), sustained very low A and K indices coupled with a low SFI are the best conditions for long distance working (especially if the required path crosses either of the poles).

Besides causing auroral activity, high geomagnetic field conditions can affect the electrons in the ionosphere, reducing the maximum usable frequency (MUF).

The relationship between A and K index is shown below:

K

0

1

2

3

4

5

6

7

8

9

A

0

4

7

15

27

48

80

132

207

400


Grey Line:

The "grey line" is a band around the Earth that separates daylight from darkness.  Propagation along the grey line is very efficient.  One major reason for this is that the D layer, which absorbs HF signals, disappears rapidly on the sunset side of the grey line, and it has not yet built upon the sunrise side. Ham radio operators and shortwave listeners can optimize long distance communications to various areas of the world by monitoring this band as it moves around the globe.
 

Understanding propagation numbers

How To Read Propagation Numbers

The A index [ LOW is GOOD ]

  • 1 to 6 is BEST
  • 7 to 9 is OK
  • 11 or more is BAD

Represents the overall geomagnetic condition of the ionosphere ("Ap" if averaged from the Kp-Index) (an average of the eight 3-hour K-Indices) ('A' referring to amplitude) over a given 24 hour period, ranging (linearly) typically from 1-100 but theoretically up to 400.

A lower A-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Ap-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.

SFI index [ HIGH is GOOD ]

  • 70 NOT GOOD
  • 80 GOOD
  • 90 BETTER
  • 100+ BEST

The measure of total radio emissions from the sun at 10.7cm (2800 MHz), on a scale of 60 (no sunspots) to 300, generally corresponding to the sunspot level, but being too low in energy to cause ionization, not related to the ionization level of the Ionosphere.

Higher Solar Flux generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; Solar Flux rarely affects the 30, 40, 60, 80, & 160 Meter Bands.

K index [ LOW is GOOD ]

  • 0 or 1 is BEST
  • 2 is OK
  • 3 or more is BAD
  • 5 is VERY VERY BAD

The overall geomagnetic condition of the ionosphere ("Kp" if averaged over the planet) over the past 3 hours, measured by 13 magnetometers between 46 & 63 degrees of latitude, and ranging quasi-logarithmically from 0-9. Designed to detect solar particle radiation by its magnetic effect. A higher K-index generally means worse HF conditions.

A lower K-Index generally suggests better propagation on the 10, 12, 15, 17, & 20 Meter Bands; a low & steady Kp-Index generally suggest good propagation on the 30, 40, 60, 80, & 160 Meter Bands.