This is the 151st most frequent Icelandic word.
hennar
"Her" (possessive).
Here, 'hennar' is a possessive pronoun used to indicate possession, meaning 'her' referring to a third party.
Herra Jónsson notaði bílinn hennar.
Mr. Jónsson used her car.
'Hennar' again functions as a possessive pronoun, specifying that the book belongs to her.
Þetta er bókin hennar.
This is her book.
'Hennar' in this context indicates possession within a relationship, as in 'her friend' which shows the friend is associated with her.