The atmosphere, at the bottom of which we live, is the envelope of air surrounding the Earth and held in place by gravity.
Air is considered to be a fluid.
By fluid we mean a substance, such as a liquid or gas, that is capable of flowing or readily changing its shape. A fluid exerts pressure in all directions, but the pressure is a function of the mass of fluid above a given surface. Pressure is measured in force per unit area. In other words, atmospheric pressure is a measure of the weight of the column of air above a point. Air, unlike water, is a compressible fluid so the rate of pressure decrease as we move up in the atmosphere is not as simple as in water where pressure varies very simply with depth at a rate of one atmosphere of pressure per 33 feet or 10 meters of water.