Their-Theirs

"Their" and "theirs" are words we use when something belongs to someone, but we don’t say if that person is a boy or a girl.
For example, if someone leaves a book behind, you can say, "This is their book," to show that the book belongs to them.
If you’re talking about a group of people, you can also use these words. For example, "They found their baggage, which means the baggage belong to them.
"Theirs" is like saying "belongs to them." For example, if someone asks, "Whose is this?", you can answer, "It's theirs," to show it belongs to those people.
So, "their" and "theirs" are used to talk about something that belongs to someone or a group without saying who exactly they are.