This is the 644th most frequent Italian word.
quei
"Quei" means "those" in English.
'Quei' is used as a masculine plural demonstrative adjective to refer to specific objects (books in this case).
Quei libri sono interessanti.
Those books are interesting.
'Quei' highlights specific masculine plural objects (marks).
Non capisco quei segni sulla lavagna.
I don't understand those marks on the board.
'Quei' indicates specific masculine plural subjects (boys).