We use possessive pronouns (= присвојне заменице) to show that something belongs to somebody. They answer the question Whose?
Possessive pronouns in fact replace possessive adjectives and nouns, and can come at the beginning or the end of a sentence:
Tom is a friend of mine. (= one of my friends)
This is our room. [ This room is ours.
Whose book is this? - Hers.