This is the 18196th most frequent Russian word.
чашку
"Чашку" means "cup" (accusative case in Russian).
Here, 'чашку' functions as the accusative case of 'чашка', indicating the direct object of possession.
Я купил новую чашку вчера.
I bought a new cup yesterday.
In this sentence, 'чашку' is in the accusative case, specifying the object to be taken for use.
Не забудь взять чашку на пикник.
Don't forget to take a cup to the picnic.
'Чашку' in this context is again in the accusative case, pointing out the object of the placement action.