A restrictive clause is a part of a sentence that provides essential information about a noun. It helps to identify or specify which person, place, or thing is being referred to. For example, in the sentence "The book that I borrowed from the library is fascinating," the clause "that I borrowed from the library" is necessary to understand which book is being discussed. Restrictive clauses are typically introduced by relative pronouns like "that" or "who." They do not require commas because they are crucial for the meaning of the sentence. Omitting a restrictive clause can lead to confusion about the subject being described.