Herbert Marshall McLuhan, CC (July 21, 1911 – December 31, 1980) was a Canadian educator, philosopher, and scholar—a professor of English literature, a literary critic, a rhetorician, and a communication theorist. McLuhan's work is viewed as one of the cornerstones of the study of media theory, as well as having practical applications in the advertising and television industries.
McLuhan is known for coining the expressions "the medium is the message" and "the global village" and predicted the World Wide Web almost thirty years before it was invented. Although he was a fixture in media discourse in the late 1960s, his influence started to wane in the early seventies. In the years after his death, he would continue to be a controversial figure in academic circles. With the arrival of the internet, however, there was renewed interest in his work and perspective.from Wikipedia
Hence in Understanding Media, McLuhan describes the "content" of a medium as a juicy piece of meat carried by the burglar to distract the watchdog of the mind. This means that people tend to focus on the obvious, which is the content, to provide us valuable information, but in the process, we largely miss the structural changes in our affairs that are introduced subtly, or over long periods of time. As society's values, norms and ways of doing things change because of the technology, it is then we realize the social implications of the medium. These range from cultural or religious issues and historical precedents, through interplay with existing conditions, to the secondary or tertiary effects in a cascade of interactions that we are not aware of.