This is a "Yellow Ribbon Campaign" (노란리본 or 노란리본 달기). The text says 하나의 작은 움직임이 큰 기적을 which means something like:
- One's smallest movements can make a huge wonder, or
- The smallest steps can have a huge impact, or
- If we all do a little, we can do a lot, or
- The littlest things can make the biggest difference, etc.
Source: 카톡 노란리본…‘노란리본 달기’ 확산 “하나의 작은 움직임이 큰 기적을”
Another one that seems to be popular is this:
This one features the same text as above, but adds at the bottom 다시 돌아오기만을 기다리며 기원합니다 which is something like "Hoping for your return" or "We're waiting for you to come back" "Wishing for you to come back again."