The document discusses defining and non-defining relative clauses. It defines relative clauses as clauses that begin with relative pronouns like who, which, that, etc. and explains that defining clauses provide essential information about the noun they refer to, while non-defining clauses provide extra, non-essential information. It provides examples of defining and non-defining clauses and discusses the proper use of relative pronouns, prepositions, and commas with defining and non-defining relative clauses.