Why The A-index is a daily figure derived from the eight 3-hour K-index readings at a magnetometer station, converted to a linear scale and averaged. Because it summarizes a whole day (and is often tracked over days), it describes the longer-term stability of Earth's geomagnetic field rather than a momentary disturbance. Low A-index values, roughly under 10, mean quiet geomagnetic conditions and better HF propagation, especially on polar paths.
Watch out The K-index is the short-term measure, updated every three hours; and the solar flux index is a radio noise measurement at 2800 MHz, made at Penticton, British Columbia, not a geomagnetic number.
A for Average over a day (long term), K for Quick 3-hour snapshot. Low numbers = quiet = good HF.