This is the 16th most frequent Norwegian word.
den
"The" or "it" (definite article or pronoun, depending on context).
Here, 'den' is a pronoun referring to a definite noun ('boken'), and it must agree with the grammatical gender and number of the noun.
Jeg ser boken, og den er rød.
I see the book, and it is red.
In this sentence, 'den' is used as a demonstrative pronoun accompanying an adjective ('nye'), agreeing with the implied noun ('filmen').
Hva synes du om den nye filmen?
What do you think of the new movie?
Here, 'den' is functioning as a relative pronoun introducing a clause ('som snakker høyt') and stands in for an unspecified entity.