An adjective clause is a group of words that describes a noun, providing more information about it. These clauses usually begin with relative pronouns like who, whom, whose, which, or that. For example, in the sentence "The book that I borrowed was fascinating," the clause "that I borrowed" describes the noun book. Adjective clauses can be essential or non-essential. An essential clause is necessary for the sentence's meaning, while a non-essential clause adds 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.