This is the 2872nd most frequent Polish word.
litewska
"Litewska" means "Lithuanian" (feminine singular form) in English.
The word 'litewska' is an adjective and here it describes the noun 'kulturze', which means 'culture', specifying that it is related to Lithuania.
Ta książka opowiada o litewskiej kulturze.
This book is about Lithuanian culture.
The word 'litewskich' is an inflected form of the adjective and here it describes 'turystów', or 'tourists', indicating their origin from Lithuania.
Spotkaliśmy grupę litewskich turystów w mieście.
We met a group of Lithuanian tourists in the city.
The word 'litewskiej' modifies 'narodowości', the noun for 'nationality', emphasizing the Lithuanian connection.