Definition and simple explanation:
air pressure is the atmospheric pressure (hydrostatic pressure) of the air on the earth's surface. As physical units bar and hectopascal (hPa) are common.
The main constituents of air in the Earth's atmosphere are nitrogen (78%) and oxygen (21%). Argon, carbon dioxide, methane and some other trace gases account for only a very small part. Although we do not feel it, one liter of air weighs 1.3 grams. Because the chemical elements of which the air is made, like all other substances have a mass, although the mass density is of course very low compared to a solid.
To imagine the air pressure easier, we create the following scenario: A person stands on a 1x1 meter area. Above it is an air column to the end of the earth's atmosphere, approximately 10km high. Air weighs 1.3 grams per liter. A cube with the edge lengths of one meter each weighs 1.3 kilograms. In humans, the weight of the entire air column affects up to 10km in height. We have to take into account that the density of the particles decreases with increasing height. People also say: "The air is getting thinner". If we add the weight of the whole air column, we reach a weight of 10,000kg per sq. M. So humans have 10 tons of weight. The cause is gravity (gravitation / gravitational force), which applies to molecular air particles as well as to humans.
With increasing altitude, the air pressure continues to decrease. For example, if we are standing In the Himalayas, the air column above us is much shorter than if we were standing at the foot of the mountain. In addition, as already mentioned, the lower particle density with increasing height.
The air weighs a bit, can be detected with a simple balloon and a fine balance. First, you weigh the deflated balloon. After the balloon has been blown up, it is weighed again. The difference between the two values is the weight of the air in the balloon.