You use hers to indicate that something belongs or relates to a woman, girl, or female animal.

In written English, hers is sometimes used to refer to a person without saying whether that person is a man or a woman. Some people dislike this use and prefer to use 'his or hers' or 'theirs'.

Hers is sometimes used to refer to a country or nation.

People sometimes use hers to refer to a car, machine, or ship.