In the South-Korean drama Crash Landing on You, a successful business woman from Seoul accidentally crash lands in North Korea. What is curious is that even though the series is in Korean, at several points throughout the movie Korean captions are displayed:
This is not a full captioning, throughout the course of the initial 70 minute episode there's only about a dozen captions like this.
From inspecting the Korean SDH subtitles, I found out that the bold characters always seem to refer to a word from the spoken dialog. By running the caption through a translate app, it seems that the captions offer a description on that specific word.
But why is this necessary? What is so special about those words that they require additional explanation?
I have a suspicion, but would like to hear an answer from someone more familiar with the cultural background.