A diagram of a manometer is shown below.
A manometer is a simple instrument used to measure the pressure of an enclosed or contained gas.
The liquid mercury at the bottom of the tube is in static equilibrium. When the gas is allowed to flow out under its own pressure, it will "push" against the mecrury.
The exerted pressure on the mercury causes it to move a certain height. The change in height is equivalent to the pressure of the contained gas.
A difference in heght of 1 cm is equivalent to 100 Pa of pressure at STP (Standard Temperature and Pressure).
This difference in pressure is called te "Gauge Pressure" of the manometer.
Since the pressure of the gas is measured against air pressure, i.e "atmospheric pressure", we have to take this factor into account by writing the following:
Absolute Pressure = Gauge Pressure (from manometer) + Atmospheric Pressure