Well, back in the olden days, a long time ago, there were lots of different groups of people who lived in what's now Europe. Some of these groups wanted to have a leader who was in charge of everyone, like a king or queen.
The Romans were one of these groups, and they ruled a big empire that included Italy, Spain, and many other parts of Europe. They had a special leader called an emperor who was in charge of all their armies and the government.
Now, the king of the Romans was a bit different from an emperor. You see, a long time ago, there was a group of people called the Germans who lived in what's now Germany. They were divided into lots of little kingdoms ruled by different kings, but they wanted to have one king who was in charge of all of them.
So, they came up with a plan: they would pick someone to be the "king of the Romans" who would be the leader of all the German kings.
This king would be chosen by a group of important people called the electors, who were kind of like the people who vote in an election. They would meet together and pick someone to be the king of the Romans, and then that person would be in charge of all the German kings.
It's kind of like if you had a big group of friends who all had their own toys, but they all wanted one person to be in charge of all the toys. So, they would pick someone to be the "king of the toys" who would be in charge and make sure everyone played nicely.
Does that make sense, little one?