The atmosphere is composed of molecules that have mass.  The weight of the overlying molecules exert pressure at the surface.
Gravity acts to crowd the molecules close to the Earth's surface.  As you move higher in the atmosphere the number of molecules above decreases, hence pressure decreases.
 
 

Temperature has an effect on pressure:

As air is heated the molecules move faster and away from one another, the air expands.
As air is cooled the molecules move slower and closer to one another, the air contracts.
 
 

 
  
 

In meteorology the absolute pressure in any one location is not as important as the difference in pressure between two locations.  A scale is added to the column of air above.

 

Vertical pressure gradients are in millibars. The drawing depicts the 500 mb isobar (line of equal pressure).  If this column is heated and cooled the pressure gradients stretch and compress accordingly.

 

At the same elevation (12,000) the pressure in column A is greater than in column B.  Because of this pressure difference air will move from column A to column B.  It is the differences in pressure that causes air to move.

Because air is moving away from A and into B the pressure at the surface will fall at A and rise at B.

Upper atmospheric pressures are important for predicting the movement of air and gives a "big picture" of the weather.  For this reason meteorologists plot the variations in pressure across a map.

Weather maps depict upper level air pressure as a constant pressure surface.  The 500mb pressure surface is commonly used.  Weather balloons and radiosondes determine the height of the 500 mb pressure gradient.  These heights are plotted on a map and contoured (isobars) much like a topographic map.  The undulating constant pressure surface reflects areas of high and low pressure: high pressure ridges andlow pressure troughs.

The isobaric surface is used to predict air movements and will become very important in discussions on storm development and wind patterns.