This is the 2792nd most frequent German word.
polnischen
"Polnischen" means "Polish" (relating to Poland or its people, language, or culture).
In this sentence, 'polnisches' functions as an adjective modifying the noun 'Buch' to describe its origin as Polish.
Ich habe gestern ein polnisches Buch gekauft.
Yesterday, I bought a Polish book.
Here, 'polnische' is an adjective describing the noun 'Küche' (cuisine), indicating it as belonging to Poland.
Die polnische Küche ist bekannt für ihre leckeren Gerichte.
Polish cuisine is known for its delicious dishes.
In this instance, 'polnisches' describes 'Erlebnis' (experience), signifying that the experience is related to Poland or Polish culture.