Learner's definition of THEIRS
that which belongs to or is connected with them
The computer is theirs. [=the computer belongs to them; it is their computer]
He bought his car on the same day his parents bought theirs.
She is a friend of theirs. [=she is their friend]
That dog of theirs [=their dog] is always barking.
his or hers
— used to refer to a single person whose sex is not known or specified