"Their" is a possessive pronoun used to indicate ownership or association. It refers to something that belongs to or is related to a group of people or a person whose gender is not specified. For example, in the sentence "The students brought their books," the word "their" shows that the books belong to the students. In modern usage, "their" is also commonly used as a singular pronoun to refer to someone whose gender is unknown or to be inclusive of all genders. For instance, "Each person should bring their own lunch" implies that everyone, regardless of gender, should bring a lunch.