It’s a famous phrase coined by McLuhan. He means that the form of a medium determines the kind of overall message it delivers. A case of scrolls carries a different message from a bound codex collecting those same scrolls, and so on. Whatever the hypothetical ability to deliver the same messages over books, TV, silent films, talkies, YouTube shorts, tweets, radio, handwritten letters, emails, et c., in practice the media themselves shape the messages that they deliver, so the broader “message” they effect in the form of shaping society and public life are very different.