It has been observed clinically that a person with COVID-19 can have very low oxygen levels but otherwise appear well, termed "happy hypoxia." It's concerning because these patients may be more significantly ill than they feel, certainly warranting closer attention in a medical setting.
If a person with COVID-19 monitoring his or her clinical status at home will want to ensure that the SpO2 reading stays consistently at or above 90 to 92%. If the number consistently drops call your doctor immediately