Looking at Wikipedia, you read that in photography, bokeh is “the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens.” In general the out-of-focus area of an image is the background, and one of the best ways to make this out-of-focus whilst retaining focus on your subject is to open up the lens and shoot at a low f-stop. It seems a bit backwards, but the lower the f-stop the more open the lens and the better chance you have of making the background blur. Getting close to your subject and have the lens opened up will give you the best chance for a blurry background. The shot at the start of this post was taken at f2.8 with a 200mm lens in an attempt to isolate the bloom from the foliage behind.
The rose image was taken with an 85mm lens wide open and this shot, taken outdoors, was a real challenge. When you open up the lens to make the background blurry, you also reduce the depth of focus. In the case of the rose, the gentle breeze in the back garden would cause the rose to move in and out of focus. Depth of focus on the 85mm at f1.4 is very shallow indeed. Hard to believe about 2m behind the rose is the door to the garden shed. This is one big advantage of blurring the background. Distracting objects behind your subject will disappear into a nice, soft, buttery blend of colour.
The last shot in this post was again taken with a 200mm lens at f2.8. Again, problems with the breeze made focus a challenge, but I liked the background colour, hence opening up the lens.