An adjective clause is a group of words that describes a noun in a sentence. It usually begins with a relative pronoun like who, whom, whose, which, or that. For example, in the sentence "The book that I borrowed was interesting," the clause "that I borrowed" describes the noun book. These clauses provide additional information and help make sentences more detailed. They can be essential, meaning they are necessary for the sentence's meaning, or non-essential, where they add extra information but can be removed without changing the sentence's core meaning. For instance, "My sister, who lives in New York, is visiting" includes a non-essential clause.