The document discusses relative clauses, which are parts of sentences that begin with relative pronouns like who, which, that, where, when, and why. It explains the difference between defining and non-defining relative clauses. Defining clauses provide essential information to identify a noun, while non-defining clauses provide extra, non-essential information. The document provides examples of forming relative clauses and choosing the correct relative pronouns, and emphasizes the importance of punctuation in non-defining clauses. It concludes by noting that practice is needed to fully understand relative clauses.