A secondary strand of the plot that is a supporting side story for any story or the main plot. Subplots may connect to main plots, in either time and place or in thematic significance.
Subplots often involve supporting characters, those besides the protagonist or antagonist.
Types of Subplots
Love - When two protagonists or supporting characters, have a secret romantic relationship.
Origins - When a protagonist (whose history is unknown to him/her/it), is given an insight into their past (it can be just a little bit, or alot). The most common way to do this is to use flashbacks.