Please explain what a Diplomat American Board of Internal Medicine?

Basically it means someone is 'board certified' in that specialty or area of medicine. It is the more correct term that the board advises us to use on our business cards.

[There is some variation in use, however: some use "Diplomate" (with a long a as in mate) in the sense that one has received a diploma from the ABIM certifying that one passed their examination, rather than "diplomat" (with a short a as in mat), which refers to a professional in the area of diplomacy, generally not considered a medical specialty.]