From what I gathered, it's a new corner/segment that will take place during their summer tour, most likely during the concerts (kind of like the Kick the Can and Ohmiya SK segments they currently have in between songs).
Ura Arashi roughly means "the reverse side of Arashi", and the purpose of the segment is for people to ask Arashi questions about anything at all-- world mysteries, puzzles, "why is it like that?!" questions... as long as the fans ask, Arashi will answer. The questions will be collected into a box and Arashi will pick the slips of paper out of it during their Ura Arashi corner.
Nevermind, I completely forgot that it's already July in Japan XD Either way, the link to the official site is here: