What is the plot of a book?
Since you did not specify any book in particular, let's focus on what plot means and looks like in a fiction book:
The word plot is a literary term used to describe the relation of events within a story and how they contribute to the overall outcome of the story. Most typical plots follow the same basic structure: exposition, rising action, climax, falling action, and resolution. The story begins with the exposition, which the author uses to introduce the setting and characters; then, the rising action occurs, usually a series of related events in which the protagonist (main character) of the story must work out or solve some type of conflict. The most tense and pivotal moment of the story is called the climax; it is usually a moment of truth in the story and could involve a fight, battle, or decision to be made. After climax, the following events are called falling action which helps to bring the story to the final moments, the resolution. By the time the reader reaches the resolution, the conflict has been resolved, so the author ties up or explains any loose ends to the story.
The plot of a book describes all the happenings that occurs throughout the book, which generally means the actions and the interactions between all the characters. In other words, the plot of a book is what would be left if you cut out all the extraneous descriptors and figurative language/literary techniques from an author's writing. This would leave you with very basic statements such as "He walked to the door. He rang the doorbell. She opened the door. They went to prom together". The original work may have included descriptors such as how nervous the guy was and how his hand trembled before he plucked up the courage to finally ring the doorbell; it may include similes of how the girls was as fidgety as an ant on hot coals as she was waiting for the boy to come by. However, the basic plot of the story cuts all this down to just what happened, without all the extra fluff.
A plot is the main events of a book devised and presented by the writer in an interrelated sequence.