"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 a gender-neutral term, making it suitable for referring to any group, regardless of gender. This pronoun helps to simplify sentences and clarify relationships between people and objects, such as friends, toys, or ideas.