Bokeh is the quality of out-of-focus backgrounds in a photo. I love good bokeh, and the way you get it is to use wide aperture, long focal length lenses or a combo of the two.
Exquisite bokeh: use a 135mm or 200mm f2.0:
Bad bokeh: usually a zoom 'kit' lens. In this case it was a 50mm f1.8 - the bokeh should have been good with this lens, but it's donut-like and distracting: