This is the 828th most frequent Croatian word.
njihovih
"Njihovih" means "their" (genitive or locative plural form).
Here, 'njihovih' describes 'učenika' (students) in the genitive plural form, expressing possession and agreement with the plural noun 'knjige' (books).
Knjige njihovih učenika su na stolu.
The books of their students are on the table.
The word 'njihovih' is used here as the genitive plural form of 'njihov', an adjective meaning 'their', matching the noun 'prijatelja' (friends) in case, number, and gender.
Vidio sam njihovih pet prijatelja u parku.
I saw their five friends at the park.
In this sentence, 'njihovih' modifies the noun 'kuća' (houses), which is in the genitive plural case due to its possession by 'njih' (them).