It simply means harsh or distracting bokeh, often characterised with a double line effect with straight edges that are out of focus, or a kind of onion ring or doughnut effect in out of focus highlights. For example (Edit, the effects below where created artificially but they illustrate the point):
vs
or
vs
It's worth noting however that the same lens will often have a very harsh bokeh one side of the plane of focus and quite smooth on the opposite side. Some lenses are smooth on both sides.
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