A subplot is a minor, supporting side story for the main plot. It may be connected to the main plot in regards to time, theme, or place. Often, it includes supporting characters. A subplot takes up less of the action then the main plot. The events which occur are less significant and have less impact on the overall story and characters. However, like the main plot, the subplot needs to be resolved by the end of the book.
Shorter books have few, if any, subplots. In general, it is common for novellas to not have a subplot at all, as novellas usually contain around 20,000 words. If the novella contains related short stories, a subplot could be present.
Subplots add depth and interest to the story. However, if they are not kept under control, they could outshine the main plot.