Okay, so imagine you want to tell somebody a story. You have two ways to do it: you can say it out loud or you can write it down. When you choose how to tell the story, the way you tell it changes.
The way you tell the story is called the "medium": talking out loud or writing it down. Now, the way you tell the story changes how the person listening or reading reacts to it.
That's what "the medium is the message" means. The way you tell a story changes the message. It's not just about what you say, but how you say it.
So if you tell the story out loud, the person hearing it might be distracted by other noises around them or might not remember it as clearly later. But if you write it down, they can go back to it and read it over again.
Different mediums can make the same message very different. It's like if you draw a picture and then take a picture of it with a camera. Even though it's the same thing, they look very different because they're in different mediums.
So when you choose how to tell a story, you're also choosing how people will react to it and what they'll remember about it. That's why the medium is the message.