A complement is a word or group of words that completes the meaning of a sentence. It often provides additional information about the subject or object, helping to clarify or enhance the sentence. For example, in the sentence "She is a teacher," the word "teacher" acts as a complement, giving more information about "She." Complements can take various forms, including nouns, adjectives, or phrases. They are essential in grammar because they help convey complete thoughts. In the sentence "The cake smells delicious," the word "delicious" serves as a complement, describing the cake's smell more fully.