Imagine you are playing a game with your friends where you have to run really fast from one end of the field to the other while carrying a ball. You could run all by yourself, or you could ask one of your friends to help you by running alongside you and protecting you from the other players who are trying to tackle you.
Now imagine that you are playing this game in a big fancy house with many rooms and floors, and instead of a ball you are carrying a message that is very important and needs to be delivered quickly. You can't just run down the hallways and stairs with the message because you might drop it or someone might take it from you. This is where the only running footman comes in.
The only running footman is a special person who is hired to carry messages and packages from one room to another very quickly. He is the only one who is allowed to run in the house because he is very good at it and won't trip or bump into anything. He wears special shoes with soft soles so he won't make too much noise and disturb the people who live in the house.
So when you have a message that needs to be delivered quickly, you can ask the only running footman to take it for you. He will run down the hallways and up the stairs as fast as he can, taking the shortest route possible, and he won't stop until he reaches the person who needs the message. This way, you don't have to worry about the message getting lost or stolen, and you know it will be delivered very quickly.