"Their" is for ownership. "They" is a pronoun and functions as a subject. Mereka memiliki rok-rok = they have skirts. Rok-rok mereka = their skirts.
Penggunaan their dan they itu dimana
Their itu untuk kepemilikan (=milik mereka).
They eat their apple = mereka makan apel (milik) mereka.
Pake their kok salah