This is the 15078th most frequent Polish word.
rumuńskiej
"Rumuńskiej" means "Romanian" (adjective, feminine singular or plural genitive).
Here, 'rumuńskiej' is used as an adjective that agrees with the feminine noun 'kultury' (culture) in the genitive case.
Książka opowiada o historii rumuńskiej kultury.
The book tells the history of Romanian culture.
In this sentence, 'rumuńskiej' modifies 'wioski' (village), also in the genitive case indicating possession from 'folklor' (folklore).
Podziwiam folklor rumuńskiej wioski.
I admire the folklore of a Romanian village.
Here, 'rumuńskiej' qualifies 'prowincji' (province), a feminine noun in the genitive case indicating possession.