1. The document discusses the relative pronouns "which", "who", and "that" and how they are used. 2. "That" is used for essential information where removing the relative clause would change the meaning of the sentence. "Which" and "who" are used for non-essential information where removing the relative clause does not impact the meaning. 3. Examples are provided to illustrate the difference, such as "The book that I thought I lost is on the shelf" versus "Moby Dick, which was written by Herman Melville, is my favorite novel."