A charge air cooler is used to cool engine air after it has passed through a turbocharger, but before it enters the engine. This is their main function. The idea is to return the air to a lower temperature, for the optimum power from the combustion process within the engine.
More specifically, it is a heat exchange device used on turbocharged and supercharged (forced induction) internal combustion engines to improve their volumetric efficiency by increasing intake air-charge density through isochoric cooling. A decrease in air intake temperature provides a denser intake charge to the engine and allows more air and fuel to be combusted per engine cycle, increasing the output of the engine.