Use a comma (or a pair of commas) to set off elements that are nonessential (also called nonrestrictive). A phrase or clause is nonessential if it can be removed without changing the main idea of the sentence; a nonessential element just adds a relatively unimportant detail.
Answer: A phrase or clause is nonessential if it can be removed without changing the main idea of the sentence; a nonessential element just adds a relatively unimportant detail.