We all love a photo that tells a story. In stories we talk about sub plots. Subplots can relate to the main plot and enrich in it many ways.
It can prelude the main plot and help create emotional attachment to the characters. It can contradict the main plot and provide irony. It can resonate with the main plot, making its point stronger.
In photography we have subject and background (or far plain). The background can relate to the subject, in similar ways that a sub plot relates to a main plot.
To illustrate that point I decided to use images with shaped bokeh.