A restrictive clause modifies the noun that precedes it in an essential way. Restrictive clauses limit or identify such nouns and cannot be removed from a sentence without changing the sentence’s meaning. A nonrestrictive clause, on the other hand, describes a noun in a nonessential way.
The terminology in this area of grammar can be confusing, so let’s get that out of the way. Because restrictive clauses provide key, identifying information, they are often referred to as essential clauses, and nonrestrictive clauses are also called nonessential clauses for the opposite reason.