We use possessive pronouns to refer to a specific person/people or thing/things (the "antecedent") belonging to a person/people (and sometimes belonging to an animal/animals or thing/things).

We use possessive pronouns depending on:
number: singular (e.g: mine) or plural (e.g: ours)
person: 1st person (eg: mine), 2nd person (e.g: yours) or 3rd person (e.g: his)
gender: male (his), female (hers)

Example :
1. Look at these pictures. Mine is the big one.
2. I like your flowers. Do you like mine?
3. I looked everywhere for your key.
4. My flowers are dying. Yours are lovely.