You use theirs to indicate that something belongs or relates to the group of people, animals, or things that you are talking about.
There was a big group of a dozen people at the table next to theirs.
他们旁边的那张桌子上坐了一大群人,有十几个。
You use theirs instead of 'his or hers' to indicate that something belongs or relates to a person without saying whether that person is a man or a woman. Some people think this use is incorrect.
He would leave the trailer unlocked. If there was something inside that someone wanted, it would be theirs for the taking.
他不锁活动房屋的门。如果屋里有别人想要的东西,他们就可以拿走。