It’s messages the fans send and the stars reply to through UFOtown, they can either send and get replies with their phones or on the site’s homepage. And to anticipate your question, no you cannot join if you’re not Korean (except if you actually live in Korea and have a KSSN). I hope you won’t mind us publishing this in case anyone else wonders
Edit: And to anticipate again, a KSSN is a Korean Social Security Number. And no you cannot use a fake one (the ones you get with KSSN generators) to join.