A relative clause is a part of a sentence that provides additional information about a noun. It usually begins with a relative pronoun such as who, which, or that. For example, in the sentence "The book that I read was fascinating," the clause "that I read" gives more details about the noun book. Relative clauses can be restrictive or non-restrictive. A restrictive clause is essential for understanding the sentence, like "The teacher who helps me is kind." A non-restrictive clause adds extra information but can be removed without changing the sentence's meaning, as in "My brother, who lives in New York, is visiting."