Bokeh is a term used to refer to the quality of the out-of-focus parts of an image. Different lenses have quite different looks, so for images with shallow depth of field, the look of the bokeh can become quite important. ‘Good’ and ‘bad’ bokeh can be a bit subjective, but most people usually look for smoother blur. Rougher bokeh, often called ‘nervous’ bokeh, can be good too, though.
If you want a lot of bokeh, you need a lens with a wide maximum aperture – a ‘fast’ lens. I have a Canon FL 58mm f/1.2 which can give a very shallow depth of field, and has reasonably smooth bokeh.