A nonrestrictive clause is a part of a sentence that adds extra information but is not essential to the sentence's main meaning. These clauses are usually set off by commas. For example, in the sentence "My brother, who lives in New York, is visiting," the clause "who lives in New York" provides additional information about the brother but can be removed without changing the main idea. Nonrestrictive clauses often begin with words like "who," "which," or "that." They help clarify or add detail without limiting the subject. For instance, "The car, which is red, is parked outside" indicates the car's color but does not define which car is being referred to.