This is the 8th most frequent Malay word.
mereka
"They" or "them."
In this sentence, 'mereka' is used as a subject pronoun meaning 'they', referring to a group of people performing the action of going on vacation.
Mereka akan bercuti ke Jepun bulan depan.
They will go on vacation to Japan next month.
Here, 'mereka' functions as a possessive pronoun, pointing to the owner(s) of the books mentioned.
Buku-buku ini milik mereka.
These books belong to them.
In this context, 'mereka' is used as a demonstrative subject, emphasizing the accountability of the referred individuals.