Okay kiddo, so there was a long, long time ago a place called the Holy Roman Empire. It was like a country but it was made up of a bunch of smaller countries all ruled by one main person, the emperor.
Now, the emperor couldn't be everywhere at once to make sure all the little countries were doing what they were supposed to do. So, he had some people called imperial vicars who helped him out.
These imperial vicars were kind of like assistants for the emperor. They would help make sure everyone was following the rules and behaving themselves.
Think of it like when you play a game with some friends and mommy or daddy are the referee to make sure everyone is playing by the rules. The imperial vicars were like the referees for the Holy Roman Empire.