Diffusers work in conjunction with other aerodynamic components of a car to help produce downforce. Because a car displaces air as it moves forward, the air molecules in the rear of the car get disrupted causing a vacuum of low pressure air.
To help channel the high speed air flow coming from underneath the car, diffusers redirect the high speed air flow to smoothly transition into the low pressure vacuum in the rear. This encourages smoother air flow underneath the car and allows better performance from other aerodynamic components such as aerodynamic wings.