D-BOX is to movie theaters what vibrating controllers are to video games :)

Or, you've probably actually experienced a "d-box" type of setup in a theme park, on those motion rides. Basically a vibrating (very sensitive to dramatic), pivoting seat. Here, a couple of rows in special theaters are equipped with that added 'experience' sent along with the film reel.

Ultimately, it's nothing all that special... but for a movie like Pacific Rim - imagine in the high intensity adrenaline pumping massive scale monster fights, feeling every enormous pounding impact from a punch; like your clench in mimicking the action brought to life for your whole body.
Or something like that... hehe

It's a mild bonus that could be appropriate for some movies (about as enhancing to a movie as a good vibrating controller is to a game), but IMO isn't worth the extra price of admission (could be up to $10 more in places). Thankfully Cineplex has the Scene program here, so if I want to see a movie in a D-Box, I'll save up 1000 points (as normal) and redeem that to get the extra add-on for free. Sweet deal.