A possessive pronoun is a pronoun that shows who owns something.

Delilah read her book.

Delilah said you can't take that book. It's hers.

The pronouns 'her' and 'hers' both tell us that the book belongs to Delilah, but how do we know which one to use?

In the second example, we use 'hers' because the pronoun is on its own and we are not mentioning the book itself.?

Look at this list of possessive pronouns:

my book / mine

your book / yours

his book / his

her book / hers

our book / ours

your book / yours

their book / theirs

Did you notice that the pronouns in the second group all end in 's' except 'mine'? The pronoun 'his' is the same in both groups because it already ends in 's'.