Their is a possessive pronoun that shows something belongs to or relates to more than one person. When you say “The students left their backpacks in the classroom,” you mean the backpacks belong to the students. When you say “Sarah and Tom brought their dog to the park,” the dog belongs to both of them together.

People often confuse their with two other words that sound exactly the same but mean different things. There refers to a place (“Put the books over there”), while they're is short for “they are” (“They're going to the movies”).

You might also hear their used when talking about one person whose gender you don't know or when someone prefers not to be called he or she. “Someone left their jacket” sounds more natural than “Someone left his or her jacket,” even though English teachers once taught that the second version was more correct. Language changes over time based on how people actually use it.