It's the feminine singular form of the possessive pronoun "le leur" (theirs), and it is used when referring to something that belongs to multiple people or things.
Here are some examples:
* La maison est la leur. (The house is theirs.)
* J'ai vu la voiture, la leur. (I saw the car, theirs.)
* Les clés, ce sont la leur. (The keys, those are theirs.)