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'.