This is the 207th most frequent Finnish word.
joiden
"Joiden" means "whose" or "of which" (genitive plural form).
In this sentence, 'joiden' is a relative pronoun introducing a relative clause, referring back to 'ystävänsä' (friends), showing possession.
Hänen ystävänsä, joiden kanssa hän vietti loman, olivat erittäin ystävällisiä.
Her friends, with whom she spent the vacation, were very friendly.
Here, 'joiden' acts as a relative pronoun relating to 'kirjat' (books), indicating possession of the 'aiheet' (topics).
Kirjat, joiden aiheet käsittelivät historiaa, olivat erittäin mielenkiintoisia.
The books, whose topics covered history, were very interesting.
In this example, 'joiden' functions as a possessive relative pronoun linked to 'lapset' (children), highlighting their 'syntymäpäivät' (birthdays).