Gas Pressure / Atmospheric Pressure
What is Atmosphere
Atmosphere is layers of gas that surrounds the Earth. So atmospheric pressure is pressure due to these layers of gas.
Take note that the gases are more denser near the surface compare to the ones that are further away from the surface. (higher altitude)
How does Atmosphere Cause Pressure
So now we know that atmosphere is just the layers of gas. Gas particles has mass. And gravity acts on all the mass. Pulling all this masses towards the Earth. Crushing or pressing everything along its way.
The higher we go, the less air there are difficult to breathe. Gravity is stronger closer to earth so more particles are pull towards so more denser near the surface compare to higher altitude.