It's just as the title shows. The story is, one of the actors from Hwarang went to one of the middle schools here in the US. He introduced himself and mentioned some of the people that he acted with.
One of the students recognized D.O from one of the dramas but not the other ones the guy was in, but he (the student) briefly mentions that he's a fan of BTS.
So, the actor randomly pulls out his phone and calls V, and he video chats with him. You can pretty much see the students over excitement and his inner fanboy came out.
I thought it was really cute and thoughtful for the guy to do that for one of Taehyung's fans. I was excited as much as the kid was XD