Hello Eunjae,

I believe this is covered in the lecture video but the focus is really on the word "line", not the whole expression "Just a line."

"A line" literally means 한 줄, 한 마디. So in the context "Just a line to let you know that...." we are really saying "간단하게 한 마디만 말씀드리자면..."

I wouldn't consider it as a common expression or anything but definitely not something that people wouldn't understand in my opinion.

I hope it makes sense!

Best regards,

Mason U, CMI-Korean
Business Interpretation and Translation in Korean (BIT-K) Instructor