There can be used as an adjective of place:-
There can also be used as the introductory subject
They're is the contraction of "they are".
Their is a possessive pronoun like "her" or "our" indicating that something belongs to more than one person, not including yourself.
Example: "There's a large family in this town. Look they're over there by their car."
!Note - If you've written "they're," ask yourself whether you can substitute "they are." If not, you've made a mistake. There" has "here" inside it to remind you it refers to a place, while "their" has "heir" buried in it to remind you that it has to do with possession.