Barg pressure is the pressure, in units of bars, above or below atmospheric pressure. The "g" at the end of the word indicates that the measurement is not absolute pressure, sometimes indicated by bara. According to The Engineering ToolBox, standard atmospheric pressure is, by definition, 1.01325 at sea-level and 0 degrees Celsius.
Physics defines pressure as force per unit area. Atmospheric pressure is due to gravity acting on the air that surrounds the Earth. This is the reason standard atmospheric pressure is defined at sea level. As elevation increases, there is less pressure due to the decrease in the weight of air.
According to HowStuffWorks, "Humans cannot live unprotected at pressures greatly above or below atmospheric pressure." Pressure also affects the boiling point of water, so recipes require adjustments for high altitudes. Underwater, the weight of water acts on a body to increase pressure, so it rapidly increases. Pressure acting on materials deep within the Earth helps to form metamorphic rocks.
Hydromechanics, the study of the effects of force on fluids, involves the study of pressure. According to Bernoulli's Principle, streams of both liquids and gases reduce pressure. According to Pascal's Law, changes in pressures transmit to all parts of a confined fluid while holding a constant temperature.