A jocular, non-serious way of telling someone that they are sucking at something (usually surfing or skateboarding or some such activity). The expression originated among Los Angeles area surfers who would try to intimidate surfers from farther inland (usually the San Fernando Valley) into not surfing at “their” break. Locals could tell a “Val” because people from the valley usually weren’t very good (not having access to the beach as easy as coast residents). Hence the expression becoming synonymous with “you suck.” Was popularized as a catch phrase by several bad Hollywood surf movies (eg Point Break).
Dude 1 tries to kickflip off a curb and falls on his face.
Dude2: Hahaha, go back to the valley.