Final answer:
Subplots are separate storylines related to the main plot that provide additional depth and relief. They involve supporting characters with their own conflicts.
Step-by-step explanation:
Separate stories or series of incidents going on at the same time as the main plot, but related in some way are known as subplots. Subplots are secondary storylines involving supporting characters that may have their own conflicts to resolve. They provide additional depth, background, and relief from the main plot. For example, in Shakespeare's Romeo and Juliet, there is a subplot involving Mercutio and Tybalt that leads to a fight and eventually affects the main conflict.