This is the 3953rd most frequent Korean word.
우유를
The Korean word "우유를" means "milk" (as the object of a sentence).
In this sentence, '우유를' (milk in accusative case) functions as the direct object of the verb '마셨어요' (drank).
나는 우유를 마셨어요.
I drank milk.
Here, '우유를' (milk in accusative case) is the direct object for the verb '먹었어요' (ate/had).
그는 아침에 우유를 먹었어요.
He had milk in the morning.
In this context, '우유를' indicates the object to be purchased, directly tied to the action of the verb '사러 가요' (going to buy).