A summary is basically a recap or breif exploration of something. So if you were summarizing a book you would explain who the main character is, who the antagonist is and give a short explanation as to what the protagonist did to stop the antagonist.
Short example:
Mary had to steal the book back from the bad guy before he used it to destroy the Earth. She snuck around his base and eventually got to the main room where she had to fight the bad guy and escape with the book before his henchmen could catch up.
