Pressure is an important property (state function) of a gas (or any substance for that matter). Pressure is a measure of the force per area that substance exerts on its surroundings. Technically it is the force per area applied in the direction perpendicular to the surface of the object.
There are a number of ways to measure pressure. These include instruments that directly measure the force of the gas, to those that detect the number of gas particles and convert this to a pressure. Knowing about how pressure is measured can help with some of the unusual units that are used for pressure. The "best" unit for pressure is a Pascal (Pa). This is a force of 1 Newton (N) per square meter (m2). However, it turns out that this is a small pressure compared to the typical values we encounter in chemistry. The most typical pressure is the prevailing pressure of the atmosphere. This pressure varies day to day, but at some point an standard unit of 1 atmosphere (atm) selected. 1 atm = 101,325 Pa. As such, atm is a historic unit that still gets a lot of use, however it is not easily converted to kg, m, and seconds. A "better" unit is the bar. 1 bar is chosen to be close to 1 atm and is defined as be equal to 100,000 Pa or 100kPa. 1 atm = 1.01325 bar.
Historically pressure was measured in a barometer (or manometer). This instrument could measure the pressure by measuring a differential in a force applied to a liquid. For a barometer, the liquid is exposed to the atmosphere and is forced up into a long sealed tube. The height that the liquid rises in the tube is then a measure of the force applied from the atmosphere. The height depends on the pressure applied and the density of the liquid such that:
\[P = {\rho gh}\]
Where g is the gravitational acceleration constant, P is the pressure, and \(\rho\) is the density of the liquid. If the gravitational acceleration constant has units of m s-2 (9.8 m s-2) and the density is in kg m-3 the pressure will be in Pascal. Because most barometers employ mercury as the liquid (since it is dense and has a low vapor pressure) the height of the liquid is often used as a measure of pressure. Thus millimeters of mercury (mmHg) or centimeters of mercury (cmHg) are sometimes used as units for pressure. Torr is another historical name for mmHg. 1 mmHg is approximately 1 Torr. Torr are strictly defined such that 760 Torr = 1 atm.