◊Their is known as a possessive pronoun because it’s used as a substitute for a more specific group of people who possess something. It is different from there, which is a preposition used to indicate direction or location, and they’re, which is a contraction meaning they are. Notice these correct uses of the above terms:
◊ We are about to pass their house.
◊ I haven’t been there in many, many years.
◊ They’re supposed to be finished with the job by 5 p.m.
◊ If we let them work on our property, the liability for accidents will be theirs.
◊ You can set the boxes down over there.
◊ What do you suppose they’re protesting?
Here are some other possessive pronouns:
my
his
her/hers
you/yours
our/ours
its