This is the 2020th most frequent Russian word.
хорошую
"Good" (feminine, accusative case).
The word 'хорошую' is the feminine accusative singular form of 'хороший', meaning 'good'. It agrees with the noun 'книгу' (book), which is feminine and in the accusative case as the direct object of the verb 'купила' (bought).
Я купила хорошую книгу.
I bought a good book.
Here, 'хорошую' agrees with the noun 'еду' (food), also feminine accusative singular, describing the quality of the food.
Она приготовила хорошую еду.
She prepared good food.
In this sentence, 'хорошую' describes 'картину' (painting), a feminine singular noun in the accusative case, indicating that the painting found is of good quality.