From the wiki:
With the Bokeh Image node you can generate a reference image for blurs.
What are some example uses for this node?
The bokeh Image controls the shape of the out-of-focus area for the Bokeh blur node.
Generally, octagonal and circular shapes are very common for most camera lenses. If no bokeh image is supplied, then a circular bokeh is assumed.
bokeh is lens blur. the fun thing about it though is how the cameras aperture changes the shape of points of illumination to mimic that of the camera. of course blender doesnt have a real camera so we have to fake the effect. it can be done in the lense settings but it is often more convienient to use the nodes.