This is the 257th most frequent Dutch word.
jullie
"Jullie" means "you" (plural) in English.
In this sentence, 'jullie' is used as the plural form of 'you', addressing a group in an inclusive manner.
Jullie zijn hier welkom.
You all are welcome here.
Here, 'jullie' is employed to indicate possession, referring to books that belong to the group being addressed.
Ik heb jullie boeken meegenomen.
I brought your books with me.
In this context, 'jullie' serves as the subject of the sentence, referring to the group expected to sing.