Bokeh is the out of focus area in a photograph, usually the background. The above image differs from most in that the out of focus area is in the foreground.
Good bokeh would be considered pleasing or add to an image. Poor bokeh is often partially in focus and can distract from the main subject. There are many things that can affect bokeh. Back in the old days, we worried about DoF (Depth of Field) which was simply the area in focus. By adjusting the lens opening, you can change how much of your subject will be in focus. Your lens openings are measured in f/stops. The smaller the number the faster the lens, and the larger the opening. Another way to think of it is, the bigger the number the more in focus. Nowadays the talk is about the bokeh, how well the lens treats the out of focus highlights.
Generally speaking, the closer you are to the subject and the longer the focal length you choose, will increase your out of focus area. Combine that with a wide lens opening, and you are on the path to good bokeh. Some lenses produce better looking bokeh than others. You can read the reviews on the web and get a wide range of opinions on how good the bokeh is for a particular lens. Faster lenses usually are considered better for good bokeh. Many lenses produce their best bokeh when wide open. This is because a smooth circle will give you the smoothest highlights in the background. My oldest lenses, a 50mm f/1.4 AIS and a 60mm f/2.8 AF-D Micro both have 7 bladed irises and you can count the sides of the circles they create. My 24-70mm f/2.8 and 70-200mm f/2.8 both have 9 bladed irises that have curved blades. The curve helps smooth the circle created by the iris, giving you smoother highlights.
The following images were taken on Aperture Priority without changing the focus.
f/16 the willow twig is entirely in focus. The background branches can be seen and are quite contrasty.
f/8 the tip of the twig is slightly out of focus. You can barely define the branches and the background is somewhat smoother.
f/2.8 the tip of the twig is blurry and the front of the catkins are starting to go out of focus. The background has a fairly even, creamy look and you can’t tell there are branches back there.
All images in the post were taken with a Nikon D300s with a Nikkor 70-200mm f/2.8 on Sandisk Extreme IV CF media.