This is the 12340th most frequent Korean word.
단추를
"단추를" is "button" (as an object) in its grammatical form indicating action or relation.
In this sentence, '단추를' is the object of the verb '잠갔어요', indicating the button that was fastened.
나는 단추를 잠갔어요.
I buttoned the button.
Here, '단추를' represents the button to be pressed to trigger an action.
단추를 눌러야 문이 열립니다.
Press the button to open the door.
In this case, '단추를' indicates the button that was accidentally dropped.