A standard atmosphere is when the temperature is 15 degrees Celsius (59 degrees Fahrenheit) and barometric pressure is 29.92 Hg.
The standard atmosphere is used with reference to weather products. The standard atmosphere does not exist very often in nature but allows for comparison to a set reference or standard. The standard is then adjusted to give the pilot the proper information for the current weather conditions.
FAA AC-00-6B Aviation Weather pg. 1-6