Bokeh or boke, in its originally is a Japanese word (though many out there might not agree with that, but that’s not our main focus here) and it means haze or blur.
In photography, the word “bokeh” is associated with the blur that is seen in the out of focus areas of an image (the background or the foreground of an in-focus subject or scene).
This is useful when you want to make the main subject of your photo standout amongst any distractions passing on around it directing all the attention in your frame towards it bokeh.
You focus upon your main target and anything else within your frame is thrown out of focus (unless you’re using a very narrow aperture opening of course, in which case everything in the image would look sharp, crisp and in-focus).
The term bokeh in photography has become more and more sought since the late 90s. a few associate it with the round circles that are generated by out of focus light or reflection sources in a shot, though bokeh isn’t really only constraint to that. Bokeh in reality expands to encompass every image with a shallow depth of field which results in sharpening the target or point of interest in the shot (otherwise known as the focal point of the frame), and throwing everything else right out of focus.
As you might have already concluded, the “Bokeh” result is directly affected by the lens aberration and aperture opening. Lens aberration is the small optical imperfections present in every lens manufacturing (with ranging levels depending on the quality and built of the lens) causing mis-representation of lines and shapes in a frame. Aperture is the opening of the lens that controls how much light is let in from a shot scene to hit the camera film or digital sensor and get recorded as an exposure.