How does a turbo work?

A turbo uses the exhaust gases to spin a turbine wheel which is connected to the compressor wheel. As it speeds up the compressor wheel draws air in a compresses it in the compressor housing. The pressurized air is forced into the engine, increasing volumetric effiecencies and cylinder pressures which increases the torque output of the engine.

In a nutshell, it forces a higher amount of air/fuel mixture into the cylinder during the intake cycle.