This height is the height at which the force of the atmosphere pushing up on the column just balances the weight of the column. If the fluid is Hg, the height of the column that is supported by the average atmospheric pressure when measured at the surface of the Earth is 760 mm, about 30 inches. Thus both mm of Hg and Inches of Hg can be used as pressure units. If the fluid in the barometer is not mercury or the graviational field is not the same as on the surface of the Earth, the height of the column for a given pressure will be different. All pressure units can be converted to one another by application of a multiplicative factor.
Gases are compressible fluids, unlike liquids.
If you squeeze a gas, it gets smaller. A simple experiment shows that if you double the pressure at constant temperature, you halve the volume. The behavior of the state of a gas can be described mathematically by relationships between the observed properties of that gas. Robert Boyle is attributed with the quantification of the relationship between pressure and volume of a gas. This relation seems somehow mechanical in nature.