This is the 9180th most frequent Korean word.
건너편에서
"Across from" or "on the other side."
In this case, it suggests the location the letter originated from, potentially indicating a geographical distance.
건너편에서 온 편지는 내 친구로부터였다.
The letter that came from the other side was from my friend.
Indicates the presence of a girl on the opposite side, specifically across some form of boundary, such as a street or a river.
건너편에서 한 소녀가 손을 흔들고 있었다.
A girl on the other side was waving her hand.
Here it denotes the origin of the sound being across from the listener.