Bridgeman was born with partially impaired stereovision -- better known as "lazy eye" -- wherein one's eyes have a tendency to drift independently, making your brain unable to accurately process depth. But when Bruce saw Hugo in 3D, he was not only able to see the "depth" in the film, but he also came out of the theater suddenly able to perceive, in his words, "a riot of depth" in the outside world.
This sudden dramatic breakthrough did have scientific precedent. A 2011 study documenting five people with impaired stereovision who later learned to see in 3D concluded that people "were most likely to have a breakthrough if the stereoscopic images were reinforced by monocular cues like relative size and shading." In short, Hugo was so effective at creating the illusion of depth that it helped Bridgeman's mind establish the necessary synaptic connections to bridge the gap to fully-functioning stereovision.
Okay, but we still agree 3D sucks overall, right?
They really should've deleted every other pull quote on the poster and gone with "Gives the gift of sight!" Not many marketing departments downplay a miracle, Hugo PR team.