A subplot is essentially a secondary plot — or any plot that is related to, but less important than, the central plot.
What Is The Point Of A Subplot?
A subplot adds texture to any story, and makes things more interesting for the reader. If a book (or a film, or TV series, or any type of story, for that matter) only had one plot, it'd be pretty boring, however well-written it was.
I'm trying to think of books without subplots, and other than children's stories that are intentionally simple, I can't think of any! Even books I consider to be terrible have subplots.
Common Subplots
The most common type of subplot is the romantic one (Nick's relationship with Jordan in The Great Gatsby is a great example), followed by betrayal subplots and subplots involving minor characters.