This is the amount of moisture in air given in kg of moisture per kg of dry air e.g. for room air at 21oC dry bulb and 15oC wet bulb, the moisture content is about 0.008 kg/kg d.a.
This is a small mass of moisture ( 0.008 kg = 8 grams) per kg of dry air or 9.5 grams per cubic metre of air.