A turbocharger is applied to make an engine more efficient, and makes it possible to “downsize” the engine to a smaller size without any loss in performance. This is why many cylinder 6 engines have become cylinder 4 engines and so forth. A smaller engine is by definition a cleaner engine so the application of turbochargers leads to reduced emissions. Nowadays it is almost impossible in the European Automotive Industry to live up to the emission norms without applying a turbocharger. A turbocharger utilizes the large amount of energy remaining in the exhaust gas of the combustion, by using this energy to propel a turbine wheel. On the other end of the turbine shaft a compressor wheel is mounted. The spinning compressor wheel sucks in fresh air through the air filter and then blows compressed air to the engine cylinders, first cooled by the intercooler. The extra oxygen contributes to an optimized combustion inside the engine significantly improving both fuel economy and emissions.
Source : https://www.turbocharger.mtee.eu/turbo-knowledge/turbo-explanation/