Relative clauses are used to provide additional information about something without starting a new sentence. They combine two sentences by replacing a word in the second sentence with a relative pronoun like "who", "which", "that", "whose", or "whom". Different relative pronouns are used for people, animals, things, or referring to a whole sentence based on whether they are the subject or object of the clause. Exercises are provided to practice forming relative clauses using relative pronouns.