SEE: Quoted Forms because this one of them. It's a Quoted Abbreviations for -다고 하다. For -다고 합니다, it is abbreviated to -답니다. For 이다, it becomes [[-란다]]/-랍니다.

Go! Billy Korean summarizes it as it is said...

Often used when an older speaker is teaching a younger listening. End of a story, giving advice (not always).

When used without a subject, it tends to be used in stories and advice because it gives off the it is said that... feel.

1. 갈 거 란다.

   • It is said that (someone) will go.

2. 오래오래 행복하게 살았답니다.

   • They say (someone) lived happily for a long, long time.

3. 탄산음료만 마시면 배가 아플 거란다.

   • It is said that (your) stomach will hurt if (you) only drink carbonated drinks.

4. 이 공사는 내년에 끝날거란다.

   • It is said that this construction will finish next year.

5. 엄마 눈에는 내가 제일 예쁘단다.

   • In mom's eyes, it is said that I am the prettiest.

6. 사고가 나서 늦었답니다.

   • They say (someone) was late because of an accident.

7. 이게 요즘 제일 인기있는 노래랍니다.

   • They say this is the most popular song these days.

8. 오늘 점심 중에 달기만 맛있었단다.

   • It is said that only the sweet things were delicious during today's lunch.