You use theirs to indicate that something belongs or relates to the group of people, animals, or things that you are talking about.
双语例句
例：
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。
2.?（用于代替“他的”或“她的”，不指明是男是女，有些人认为这种用法不正确）
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.
双语例句
例：
He would leave the trailer unlocked. If there was something inside that someone wanted, it would be theirs for the taking.