The Solar Flux Index (SFI) is a gauge of solar particles and magnetic fields reaching our atmosphere. Higher numbers mean more solar wind is reaching the earth.
The K-Index (or Boulder K)is a gauge of geomagnetic activity relative to an assumed quiet-day curve for Boulder,Colorado. Falling numbers mean improving conditions particularly in northern latitudes and areas where aurora activity can occur. (The further you are from Boulder,Colorado, the less exact this information will be.)
The A-index is simply an index of geomagnetic activity derived from a scaled average of the previous 24 hours K-index readings. Use this as a reference for general conditions on the bands.