After a 1/1 bid, a reverse shows a good 15+ HCP, an unbalanced hand, and at least five cards in the first suit. (HCP strength can be lowered in cases of extreme distribution.) It is forcing and game-forcing unless Responder bids one step higher. In that case Opener will retreat to the first suit unless the distribution is particularly freaky.
All other bids by Responder are natural and forcing to game. A rebid of the first suit shows a decent 5+ cards.