The standard (click here), or near-average, atmospheric pressure at sea level on the Earth is 1013.25 millibars, or about 14.7 pounds per square inch. That probably doesn't mean that much for most folks. But, the 14.7 pounds per square inch isn't created by gravity, although gravity does act to keep the atmospheres in place of a rotating Earth.
So where does air pressure come from? From the pressure/weight of the air. I am not being a wise guy. The air pressure in pounds per square inch means that air/atmosphere has weight. Molecules have weight. Earth's atmospheres are made of molecules. So at Earth's surface, one square inch is the surface measurement, but the other measurement goes straight up. That means for a column of air above that square inch there is the weight of the molecules measures 14.7 pounds (click here). No, no one really feels that 14.7 pounds over their entire body because we are immune to that weight of air. But, it is there.
The illustration below shows how air becomes more thin as elevation increases. Hence, as the air thins, the air pressure is less as well.
