The document explains subordinate or dependent clauses, which function as single parts of speech in sentences. It details different types of clauses, such as adjective clauses (modifying nouns/pronouns), adverb clauses (modifying verbs/adjectives/adverbs), and noun clauses (acting as nouns). It also discusses the role of relative pronouns in introducing adjective clauses and the use of subordinating conjunctions for adverb clauses.