The air in our atmosphere is made of molecules of gases such as oxygen and nitrogen gas. These molecules have a mass and are made of matter, thus gravity pulls them downward towards the earth's surface. We have the weight of the entire atmosphere (from space down to us) sitting on top of us our entire lives, and the exact weight of the atmosphere at any given time is referred to as atmospheric pressure.
Normally...
The air in our atmosphere is made of molecules of gases such as oxygen and nitrogen gas. These molecules have a mass and are made of matter, thus gravity pulls them downward towards the earth's surface. We have the weight of the entire atmosphere (from space down to us) sitting on top of us our entire lives, and the exact weight of the atmosphere at any given time is referred to as atmospheric pressure.
Normally we have around 1 atmosphere of pressure on us, or 1 atm. This number can go up or down based on a number of conditions including humidity, temperature, and altitude. The higher in the atmosphere you go the less air is above you, thus the lower the pressure. Sea level is used as the standard, as all other heights are also based off of this value.
The current tool used to measure air pressure is the barometer, a large compass-like tool that can finely weigh the air and show the results as atmospheres, torr, or millimeters of mercury (mmHg for short). The first barometer, built in 1644 by Evangelista Torricelli, used a large glass tube and liquid mercury over a plate. Based on the air pressure each day the mercury would move up or down in the tube and it was measured in millimeters, thus millimeters of mercury (mmHg). The accepted "normal" air pressure at sea level is indeed about 760 mmHg or 1 atm of pressure. We may have created better technology than the large open tubes of mercury but the old standard unit still remains, even if no modern barometer uses mercury any more.
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