On Wikipedia bokeh is described as “the way the lens renders out-of-focus points of light”. Think of the the part of a photo that is out of focus. That is the bokeh. Some lenses make good looking bokeh and others don’t. Soft and even bokeh is often thought of as beautiful, while bokeh that is uneven and distracting is considered ugly.
The quality of bokeh in large part depends on the lenses that are used.