This is the 14748th most frequent Korean word.
밑으로
"밑으로" means "downward" or "underneath" in English.
In this sentence, '밑으로' is used to indicate the direction to place the book below or under the desk.
책을 책상 밑으로 두세요.
Put the book under the desk.
Here, '밑으로' describes the transition of the picture being moved to a position just beneath the wall.
그림을 벽 밑으로 옮겼어요.
I moved the picture below the wall.
In this context, '밑으로' is applied to show the direction the ball rolled to a place underneath the stairs.