A diffuser is simply a plane that has been set to have the matte translucent material. You set the transmission to almost pure white and place it between the light source and the subject. The distance between those two elements changes how the diffuser affects the light. In addition you can operate on the alpha channel to make the diffures a bit transparent, which causes the light to pass through it more easily.

The diffuser is, obviously, placed out of frame and is used to diffuse the light. In photography a diffuser is often employed to soften the harsh sun light and provide a more flattering light for the subject. 

Hope this helps.

Paolo

https://www.preta3d.com
FB: https://www.facebook.com/RealityPlugIn
Tw: @preta3d
G+: https://plus.google.com/106625816153304163119
The Reality Gallery: https://reality-plug-in.deviantart.com