"Theirs" is a possessive pronoun used to indicate ownership or belonging. It refers to something that belongs to a group of people or individuals previously mentioned. For example, in the sentence "The books are theirs," it shows that the books belong to a specific group. In English grammar, "theirs" is often used in place of a noun to avoid repetition. It is commonly used in both spoken and written language. Understanding possessive pronouns like theirs helps in constructing clear and concise sentences, making communication more effective.