A manometer is a pressure measuring instrument.
A very simple manometer is a liquid held in a U-shaped tube where the measured pressure is applied to one side of the tube whilst the reference pressure (that of the atmosphere) is applied to the other. The difference in liquid level represents the applied pressure.
A second type uses the deflection of a flexible membrane that seals a fixed pressure reference volume to determine the pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined using a lookup table.
A third variant uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm connected to the tube.
Manometers are used to measure vacuum pressures, especially in the range from 0.001 atmospheres to 1 atm. They are helpful because the deflection of the manometer is not dependent upon the type of gas being measured, unlike other types of vacuum gauges in this pressure range. The deflection of the piston is often one half of a capacitor, so that when the piston moves, the capacitance of the device changes. This is a common way (with proper calibrations) to get a very precise, electronic reading from a manometer, and this configuration is called a capacitive manometer vacuum gauge.