I don't know if I've ever mentioned bokeh before. Bokeh is the westernized spelling of the Japanese word boke, meaning fuzzy. It's used to describe the quality of the off-focus background in photographs. When you shoot with a lens wide open, i.e., at its biggest f stop, the depth of field, or the front-to-back range of sharpness, is very short. The out of focus areas are the bokeh. The faster a lens is, the large its maximum f stop, results in a shorter depth of field. High-end lenses like this also have more blades on the iris as well which add to the smoothness of the bokeh.
The picture of Betsy, above, was shot with my 70-200mm f2.8 lens. I was zoomed in at 200, and wide open at f 2.8. Focusing on her right eye, the depth of field is about an inch, from the eye to just short of the end of her nose being sharp focus. Everything else falls off into creamy smoothness.
(When shooting animals or people with a short DOF the best focal point in on the eye closest to the camera. Even if the rest of the picture is out of focus, with the eye sharp it'll look okay.)