An airside economizer is a device that brings in more outside air to provide additional cooling. It is often based on the drybulb temperature of the outdoor air. Another common control strategy is comparitive dry bulb - where the building controls determine if the outdoor air has a lower dry bulb than the return air (or has lower enthalpy), additional outdoor air enters the supply air. Airside economizers typically allow a maximum of 100% of the supply airflow to be outside air. Airside economizers are required for most buildings based on ASHRAE std 90.1
Example:
On a sunny 60 degree day, a building may require air conditioning. However, thanks to the economizer, the building AHU brought in 100% outside air and the cooling equipment did not have to run. (on a small scale, this is like turning on the vent in a car, but not turning on the a/c)