Diffusers are aero parts on the underside and rear of the car that are designed to accelerate the flow of air under the car. These diffusers are often flat with vertical 'fences' or fins and are made of metal, fiberglass, and/or carbon fiber. Their role is to direct and even force the air out to create an area of low pressure under the car. This low pressure allows the higher pressure above the car to increase downforce. This increase in downforce beyond what the tires and suspension would normally provide is known as aerodynamic grip. Increasing the aerodynamic efficiency of a vehicle will also increase fuel mileage.
Some diffusers while roughly based upon or designed to look like functional diffusers provide little to no aero benefit, but are instead designed for aesthetic purposes to finish the rear of the car or give it more aggressive styling.