This phrase is McLuhan's central thesis that the societal changes that come about as a result of media are not primarily the result of their content or use, but rather the result of their form in and of itself.
A medium, such as print or television, has content, such as news or stories or television programs. Conventional wisdom says that the content is the message; that is, that the personal and social consequences of a medium come from its content. One statement of such a view comes from General David Sarnoff: "We are too prone to make technological instruments the scapegoats for the sins of those who wield them. The products of modern science are not in themselves good or bad; it is the way they are used that determines their value." This is the viewpoint that McLuhan disagrees with.
Some media are more susceptible to have their content perceived as the message than others. Before modern art, people tended to ask what a painting was about, but they did not similarly ask what a melody or a house or a dress were about.
By saying "the medium is the message," McLuhan asserts by contrast that it is the form of the medium that has personal and social consequences, not its content. "The personal and social consequences of any medium—that is, of any extension of ourselves—result from the new scale that is introduced into our affairs by each extension of ourselves, or by any new technology."
Whereas operators of media tend to be focused on the content, owners of media tend to be more aware of the power of media, and how it has very little to do with the medium's content.
As an example, McLuhan refers to industrial automation. Automation eliminates some jobs, while creating other ones and (he asserts) giving those people more of a sense of involvement in their work. These societal consequences are a result of the technology or medium of automation, not the content of what is being produced: "it mattered not in the least whether it turned out cornflakes or Cadillacs."
McLuhan cites the physicist Werner Heisenberg who in turn cites a Chinese story in which a sage recommended to a farmer to use a mechanical irrigation system. The farmer responded that he would not use such a system because "whoever uses machines…grows a heart like a machine."