Possessives tell who something belongs to. They also tell relations between people.
Subject forms and object forms
It is my dog. The dog is mine.
Yes, it is your dog. The dog is yours.
It is his dog. The dog is his.
It is not her dog. It is not hers.
The dog wears its collar. You are sure it is its?
He is our son. Yes, he is ours.
Is he your son? I did not know he was yours.
Look, their house is small! Are you sure it is theirs?