Their is the possessive form of they, third party plural. Therefore, it should be used only to indicate plural possessive. However, their is not the only plural possessive. We also have our and your, which are the possessive forms of “we” and “you”, first party and second party plural respectively.
Ex: All of our friends’ houses are located in the northern part of the city. We met your parents at a party the other day. Their jobs are very stressful.