By reading the title I too was also confused. My first thought was WTF does that even mean! So don’t fear, Liv is here! I’m going to try my best to help you to understand how the medium is in fact the message.
McLuhan defines what the ‘medium’ is as “any extension of ourselves”. The medium therefore aids us in promoting our message rather than being the message. For example, a pen is an extension of ourselves as we use it to help us complete what we wish or hope to write or draw.
We can then understand that ‘the medium is the message’ by understanding that it is the characteristics of anything we create (the medium) that has the potential to change the effect of the message.
So basically, “the way we send and receive information is more important than the information itself” (Eudamonia video).
Eudaimonia, 2016, The Medium is the message by Marshall McLuhan, online video, December 7th 2016 ,viewed 24th of March 2020,