SUBPLOT sometimes referred to as a "B story" or a "C story" and so on, is a secondary plot strand that is auxiliary to 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. protagonist or antagonist.
THE LORD OF THE RINGS TRILOGY is an excellent example of subplots. The main storyline concerns Frodo's quest to return the ring. A secondary storyline is woven around the adventures of Aragon and Legolas' in their quest to protect the settlements and destroy the Orc armies. A third subplot concerns Pippin's and Merry's escape from the Orcs. In the end, all three subplots are woven into the main storyline.