Memorizing an immense amount of information on different medically-related subjects, integrating that information within your brain, and finally applying it correctly and promptly to the question presented (usually as a clinical vignette with some having rather long question stems). It's a Herculean task, but it can be done.
Many say that compared to the mcat there is less critical thinking and a lot more memorization involved. While the latter is certainly true, don't be fooled and think that it is solely rote memorization. It's not.
Your last question can be answered by a simple search. While there is a little correlation, there are routinely students that do relatively less optimally on the mcat but absolutely rock the boards.