Again compare how the moving marbles feel. You are bombarded by millions of particles every day. The atmosphere is a blanket-like layer of gases surrounding Earth. Gravity pulls the gases down, causing more gas molecules to be in the part closest to Earth. Thus the air near Earth has a greater density than the air at a height above Earth.
Density is a measurement of how much matter is packed into a certain volume.
The gas molecules in the air are in constant, random motion. The gas molecules are widely separated, and they collide with each other and with any other objects that get in their way. The force exerted by the gas molecules in the air on a particular area is called atmospheric pressure. As the density of the air increases in a particular area, the number of gas molecules increases, resulting in a higher atmospheric pressure. Thus the density of air molecules and air pressure are directly related.