The Earth is surrounded by a thin layer of gas called the atmosphere. Gravity pulls the gas towards the Earth, so it stays close to the surface instead of drifting away into space.
The atmosphere is made of molecules that are constantly moving and colliding with every surface, including you. These collisions produce atmospheric pressure. It acts on a surface in all directions, and always at right angles to the surface.
In the simple model the atmosphere gets thinner as you go higher: its density falls, with fewer molecules in each cubic metre of air. The pressure at the Earth's surface is due to the weight of the column of air above it, so the pressure falls as you go up. At sea level atmospheric pressure is about 100 000 Pa. You do not need to learn the layers of the atmosphere.
