Bokeh – pronounced – bo-kay is that aspect of the photograph that is blurred with the main subject is sharp focus. This is the ability of the lens. Bokehs are preferred by the pundits when you want the subject to stand out. For e.g., when you shoot a landscape, you would usually want all in the photo to be in sharp focus. But when you take a portrait, isolating the subject from the background puts the subject up more prominently. In the photo below, if the background was in sharp focus, the subject wouldn’t be standing out, but with the bokeh (quality of the background blur), it catches the eye much more prominently. .
Bokeh is also a function of the aperture. As the apertures gets larger (i.g., F3.5, F2.8, F2.0, etc), the subject tends to be in better focus. In other words, the depth of focus (DOF) is much smaller. With an aperture such as F8, most of the photo will be in good focus and such apertures are better for landscapes. But when you have to isolate something in a photograph, you need a much shallower DOF and good bokeh will add to it.