The N stands for Normal. It means it was measured at standard temperature and pressure.
Because the volume of gasses change with temperature or pressure, it is necessary to specify the temperature and pressure the flow rate was measured at.
Standard pressure is 1 atmosphere. Standard temperature varies between industries, usually 0 ºC or 20 ºC.
In the case of air compressors, I do not think it will matter much. Just use m³/hr instead of Nm³/hr. You can convert to liter/minute on the flow rate conversion page.