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The medium is the message' means that its not the specific content of new media that's important, but rather the very nature of the media itself, and the way media extend the reach of human beings in various ways.
A major concept in the book is how certain media fragment, or individualize society, while others integrate, or re-tribalize society. McCluhan's most exemplary fragmentary medium is literacy, especially alphabetic literacy, and especially alphabetic literacy after moveable type printing. He cites television, and presumably the web of electronic media that was to follow, as an integrating 're-tribalizing' medium, psychologically interconnecting all humans in a society the way old oral traditions did before societies were fragmented by the individuality of lone reading and thought.
An interesting premise. I’m not sure how the hot/cool media dichotomy ties into the fragmenting/integrating dichotomy. Perhaps I need to read Understanding Understanding Media.