A turbocharger is a small fan pump that spins around a shaft. The pump is driven by the pressure of the exhaust gas. A turbocharger consists of a turbine and a compressor. They are both mounted on the same shaft. The turbine is a heat engine. It converts exhaust heat and pressure to rotation. This rotation is used to turn the compressor.
The compressor takes in the draws in the outside air. It squeezes or compresses the air. It then sends the air to the engine. Because the air pressure has been increased, more air and fuel may be put into the cylinders. This is sometimes called boost pressure. With more fuel to burn, the engine can create more power. This increases the horsepower of the engine.