Draw six more squares next to the first one. This pressure is pushing on the paper from all sides, top, bottom, left, and right. The strength with which are pushes, its air pressure, is always changing. You can see this using a ziploc bag of air. If you increase the volume, or give the gas more space to move about, the pressure will drop.
Temperature also effects pressure.
Colder gasses tend to be made up of molecules that are moving less than those of the same gas at a warmer temperature. This means the molecules in cooler air do not move around as much, and collide less often and less violently than molecules of warmer air. Air pressure can change from one area to the next as the temperature of the air and the volume, or space it takes up, changes. When air pressure is low in one locality, air from another area will rush in to equal out the air pressure. This brings in stormy weather.