Bokeh “is the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens”. In quotes since the explanation is direct from a technical description on-line.
In a controlled studio environment we would use a combination of a wide aperture setting on a lens and have the subject stand away (forward) from the back drop or any props. Using bokeh correctly can be the difference between a good and great image.
Explain that to a wild 5 foot bird. Best we can do is get close to the bird, the heron above, and try to get an angle with any background further back. A larger zoom lens is needed.
Hard to get, except in an open marsh. This image took full advantage of that.