Milibar is the unit to measure air pressure. Centimeter and inch are also its units.
Even though we can't see air, it is real and is made of up many molecules which are zipping around at astonishing speeds. Air molecules travel about 1,090 mph at the surface of the Earth. That said, all of those molecules definitely exert a force on the objects they come into contact with.
The Earth's atmosphere has pressure and it is not constant. Atmospheric pressure is higher at sea level, and decreases as you go higher up in the atmosphere. Some weather systems have slightly higher pressure than others; weather forecasters often talk about High pressure and Low pressure weather systems.
Let's look at how atmospheric pressure is measured. Traditionally, atmospheric pressure has been measured by a mercurial barometer. The first was actually introduced in 1643 by one of Galileo's assistants. A mercurial barometer has a section of mercury exposed to the atmosphere, which exerts a force downward on the mercury (see image). If there is an increase in pressure, it forces the mercury to rise inside the glass tube and a higher measurement is indicated. If atmospheric pressure decreases, downward force on the mercury lessens and the height of the mercury inside the tube lowers. A lower measurement would be indicated. This type of instrument can be used in a lab or a weather station, but is not easily transported. Measurements from a mercurial barometer are mostly made in inches of Mercury (in Hg).
An aneroid barometer can be used in place of a mercurial barometer. It is easier to transport and is often easier to read. This instrument contains sealed aneroid cells that contract or expand depending on changes of atmospheric pressure. The contractions or expansions of the cell move a mechanical arm that indicates higher or lower air pressure (see image). An aneroid barometer is what is used in aircraft as the altimeter. The altimeter is the vital instrument that measures height of the aircraft off the ground once the altimeter is adjusted for nonstandard pressure and temperature levels.