A diffuser in an F1 car is the rear part of the bodywork. The air flowing under the car exits through the diffuser as shown in the photo. This action by the diffuser is responsible to create lion’s share of downforce.
An interesting thing about fluid moving at high speed is that the pressure will be low. So the job of the diffuser is to make the exiting air to be as fast as possible, hence reducing pressure. This pressure drop will consequently generate more downforce.