A manometer is an instrument that measures pressure. Manometers originally were constructed to measure pressure by examining the heights of liquids in a contained shape, but now manometer commonly refers to all types of pressure measuring instruments.
The simplest manometer is a thin tube shaped like a "U". The tube contains a known liquid, like water. This design measures the pressure difference between two areas, or airflows. The difference in the liquid height between the two sides could be measured, and using the density of the liquid, the pressure difference could be calculated.
Since then, many more complex manometers have been designed to measure pressure in a wide variety of situations.