A person who is magisterial can be distinguished and grand, or possibly just conceited and bossy. You will learn a lot if you listen to a magisterial presentation of early American history.
The Latin word for teacher is magister, so think of magisterial as describing a person with the great authority of a teacher or learned person. It can also mean related to the office of magistrate — think of magisterial documents or inquiries into a matter. If, however, someone calls you magisterial, he or she may think you are a bit pompous. It will irritate you if a person speaks to you in a magisterial tone!