What does that mean?
Well, when you watch at 3D movie, you are focusing on the screen, which is a set distance away - lets say 50 feet. But 3D movies attempt to create the illusion of depth, with some images appearing closer and some further away - even though they really aren't. And therein lies the problem.
Convergence is when our eyes naturally angle in to focus on a near object, or become parallel to focus on distant objects. In the real world... the focal point, and convergence point are the same. In a 3D movie, they are vastly different - and our brain and eyes have difficulty processing it.
According to Murch, and we tend to agree, it doesn't matter how advanced 3D technology becomes, it will alway cause this problem - 3D is battling 600 million years of evolution in vision. The only solution is truly holographic images that occupy real 3D space.
Check out the full article at Roger Ebert's Journal