The diffuser is an air accelerator. It has an intake, nozzle and outlet. The rear underbody is the location that produces the majority of downforce on a lot of cars. In essence the function is pretty simple, bring air in, accelerate it (reduce its pressure) and then merge it back into the air stream behind the car. In practice it can be really complicated.
If you lower the base pressure at the rear of the car where flow merges back in, you can have a huge effect on how well the whole system works. The low pressure generated by the wing is a gradient, the further away from the wings bottom surface you get, the less it effects the pressure of the air around it. The fluid is constantly trying to reach equilibrium.
Placing a wing lower and further aft can work in harmony with a diffuser and magnify its effect. Rules that require wings to be forward of the furthest aft point on the car are to contain this. You can play with a second wing lower to the deck or end plates that hang past the rear of the car, all these things are to lower the base pressure at the diffuser outlet. Hope that helps.