It means the signal (noise) level the mic is picking up is below the range you have selected. For instance, if you have selected the 70 dB range, it will register (If I recall) signals 10 dB above or below 70 dB. So if the room is quiet, the background noise is below 60 Hz and the meter will get the low indicator. If the signal is above 80 dB, it will read “HI.”
Basically,if the meter shows "HI" or "LO" you need to move the selector to the position that gets a reading.
Regards,
Wayne A. Pflughaupt