The Medium is the Message

Marshall McLuhan had said the phrase "The Medium is the Message" in the book Understanding Media: The Extensions of Man. Which was meant to mean that medium influences how the message is perceived.
When it is compared to the book and movies. A part of a book could be reread at whatever time you would like to but now movies are also able to be viewed over again without watching the whole thing in its entirety. I understand what he means about all of this but some other explanations were more over the top.
For example in part of his explanation I got a little confused. When he began to explain how a light bulb would be a good example of this saying. I understand how a light bulb can change a space because of its presence since it can change the whole atmosphere when it is illuminated, but the relationship between that and how it compares to receiving a message is what confuses me.
I don't agree entirely with this because not all people see the medium as the entire message some people actually look at the message that they are given not just how it is given.