QNH is atmospheric pressure at aerodrome level reduced to sea level according to an ISA lapse rate of temperature (1.98 deg C per thousand). This is because the thickness of the layer between two pressure levels depends on the mean temperature of the layer and you want the altimeter to read field elevation at the datum point.
As a matter of interest QFF is atmospheric pressure at aerodrome level reduced to sea level assuming no temperature change (isothermal) in the layer.
Isobars are lines joining placing of equal QFF and what you see on a synoptic chart.
Just for the sake of completeness QFE is atmospheric pressure at aerodrome level (or runway threshold in the case of "threshold" QFE).
Hope this helps.