Its how to fake a depth of field effect with no cost to rendering times. Usually if you were to do a DOF effect directly in the render you would set up your camera to work as a physical camera, add a lens shader like a bookeh effect which would simulate the depth of field you get using a real camera, but this can drastically increase you render time because the render engine has to compute the blur in real time across a 3d space. However you can just render your scene with no blur and also render out a Zdepth pass ... which looks a little like this.
What it is is a greyscale representation of the whats found in the z-axis. So the closer something is the blacker it is. Now you take this image into photoshop and paste it into the alpha layer of the original rendered image, load up lens blur from the effects list and just click on the part you want to focus on. This is what you get as a final result.
:) Check out the preview. Its as simple as that and faster than anything around and looks great. Also this is crucial if you really want to get a photo realistic image because this is one of those small details that make a good image great.