A sub-plot is a secondary plot that runs parallel to the main plot in a work of literature. It involves a separate storyline that is connected to the main plot, but deals with different characters, situations or themes.
Sub-plots are often used to provide a contrast or counterpoint to the main plot, to develop secondary characters or themes, or to add depth and complexity to the overall narrative. They may also serve to reinforce or challenge the themes or messages of the main plot.
In short, a sub-plot is a separate narrative within a work of literature that is connected to, but distinct from, the main plot.