Possessives are determinatives. They determine ownership and relationships. They are used in subject and object forms.
Subject forms are used before a noun. Object forms are used on their own.
Subject form Object form
It is my house. This house is mine.
Is that your dog? Or is this one yours?
Is it his cat? Yes, the striped cat is his.
That is not her car, is it? Yes, its hers.
The cat loves its new soft toy.
Where is our train? The one to the right is ours.
Is it your plane? I thought the one at gate nine was yours.
Look, their bus is leaving! Are you sure that bus is theirs?