This is the 14th most frequent Polish word.
które
"Które" means "which" in English.
The word 'które' here functions as a relative pronoun, introducing a clause that describes the antecedent 'książki' (books).
Znalazłem książki, które szukałeś.
I found the books that you were looking for.
The word 'które' is used as an interrogative pronoun to introduce a question about choice among multiple options.
Które z tych filmów już widziałeś?
Which of these movies have you already seen?
Here, 'które' serves as a relative pronoun to specify the subset of 'osoby' (people) being discussed.