The SFI number is the number of solar flares
Solar flux is measured in solar flux units (SFUs). It is the amount of radio noise or flux emitted at a frequency of 2800 MHz (10.7 cm, hence is it also called the 10.7 cm flux index).
When talking about the solar cycle the numbers are based on ‘smoothed’ numbers, this means the monthly average rather than the figures for each day. Using ‘smoothed’ numbers gives more accurate results, daily solar activity varies dramatically, making accurate results difficult. A and K INDEX are measurements of the magnetic field activity that are used in predictions.

The K index is a reading of the magnetic activity as compared to a ‘quiet’ day made from various locations. This index in the range 0-9. The K index is the average of of these readings.
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
The A index relates to geomagnetic stability and is calculated from K indices for the previous day.
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
A quick example of the A & K numbers
Say we are waiting for excellent conditions on the HF bands, then we are looking A and K indices of 0 lasting for a few days. Combined with a SFI (above 250). This can get all messed up by various random things happening on the sun, with these numbers you would be having a good time on HF.

If your interests are the lower HF bands, low A and K indices with very low SFI numbers would be the go.
In the real world generally propagation conditions are OK when the A index is 10 or lower, and the K index is 3 or lower and the SFI above 90. But remember lots of hams (Some like me using compromised antennas and low power) worked very good DX, added new DXCC entities to our logs, made new friends and had lots of fun. All at the bottom of the last solar cycle, when the numbers were terrible. So don’t let understanding the numbers stop you from getting on the radio. There are countless stories about surprises on supposedly dead bands.
Cheat notes (for the non readers)
The A index [ LOW is GOOD ]
· 1 to 6 best
· 7 to 9 is OK
· 11 or more not so good

K index [ LOW is GOOD ]
· 0 or 1 best
· 2 is OK
· 3 or more not so good
· 5 is really terrible

SFI index [ HIGH is GOOD ]
· 70 not good
· 80 good
· 90 better
· 100+ best
Thanks to AL (VK4BL)