I think the only help is in the tooltips. Let us know if anything is unclear.
Yes.
It is the metric that is minimized during registration. Read about the metric in journal papers, such as https://www.researchgate.net/publication/222899257_BRAINSFIT_Mutual_information_registrations_of_whole-brain_3D_Images_using_the_insight_toolkit.
Usually registration is not very sensitive to the number of bins. You can probably use the default value or slightly change it and see if it makes results better. See more details of mutual information metric in journal papers.