Its the formal greeting of "hello".

Vkook and MinMyYoongi lol same! ARMY here ! :) And yeah, you can just say it back to that person. :)

Vkook - you made me giggle with your comment! (fellow ARMY here)

you don't know anneyonghaseyo?

Annyeong-haseyo means hello in korean. So I guess you just reply with 'annyeong-haseyo' back.