A successor is a person who comes after another person in a job or a title.
A successor is usually thought of as important and powerful, because they take over from someone else.
Examples:
The new king is the old king's successor.
The successor to the president will be chosen soon.
The successor is the new boss.
The successor to the teacher is a very smart person.
The successor to the superhero will have to be very strong.
The successor to the queen is a very lucky person.