This is the 17647th most frequent Polish word.
zieloną
"Zieloną" means "green" in the accusative singular feminine form.
In this sentence, 'zieloną' is the feminine singular accusative form of the adjective 'zielony,' which means 'green,' modifying the feminine noun 'książkę' (book) in the accusative case.
Mam zieloną książkę na stole.
I have a green book on the table.
Here, 'zieloną' describes 'sukienkę' (dress), a feminine noun in the accusative case, indicating the direct object of the action.
Kupiłem zieloną sukienkę dla mojej siostry.
I bought a green dress for my sister.
In this context, 'zieloną' modifies 'sałatę' (lettuce), a feminine singular noun in the accusative case, which is the object of the verb 'zjadłem' (I ate).