This is the 439th most frequent Dutch word.
jouw
"Jouw" means "your" (possessive, informal).
Here 'jouw' is used as a possessive pronoun to denote ownership, specifying that the book belongs to the person addressed.
Dit is jouw boek.
This is your book.
In this sentence, 'jouw' indicates ownership of the advice by the person addressed, specifying whose advice was followed.
Ik heb jouw advies gevolgd.
I followed your advice.
In this question, 'jouw' is used to inquire about the ownership of the keys, referring to the person being asked.