theirs vs they

theirs

• That which belongs to them; the possessive case of they, used without a following noun. 

they

• A single person, previously mentioned, but typically not if previously named and identified as male or female, especially if of unknown or (since 21st century) non-binary gender. 

• There (especially as an expletive subject of be). 

• People; some people; people in general; someone, excluding the speaker. 

• A group of entities previously mentioned. 

• Their. 

• The, those.