A non-restrictive clause is a part of a sentence that adds extra information but is not essential to the main idea. These clauses are usually set off by commas. For example, in the sentence "My brother, who lives in New York, is visiting," the clause provides additional details about the brother but can be removed without changing the sentence's core meaning. Non-restrictive clauses often begin with words like "who," "which," or "that." They help clarify or elaborate on a noun, such as in "The book, which I borrowed from the library, is fascinating." Here, the clause enhances the sentence but isn't necessary for understanding the main point.