Atmospheric pressure is the weight of the air itself pushing inward on the airplane.  Being a gas, air can be compressed or expanded depending on the pressure on it.  At it’s very basic level, as you increase altitude, less air is weighing down on the aircraft, and on itself, thus it becomes less dense.

At sea level, air has an average pressure of 14.7 pounds per square inch, or 29.92 inches of mercury.

The effect of increasing temperature decreases it’s density.  Decreasing temerature will increase it’s density.  Humidity also decreases the density of the air.

The density of the air has an important effect on aircraft performance.

Related posts:

1. Bernoulli's Principle of Pressure
2. Altimeters (Part Three) Non Standard Pressure and Temperature
3. High Speed Flight – Mach Number Versus Airspeed