The thesis or main theme of Understanding Media by Marshall McLuhan is that media forms—audio, video, print, visual—are an extension of our functional senses (i.e. sense of hearing). For example, McLuhan posits that radio broadcasts are an extension of our sense of hearing. This puts the media in a technological or functional perspective.

The second theme is that these technological mediums, or channels of the media, create a feedback loop with the actual human senses. Essentially, we create media (e.g. television and radio broadcasts, written articles, social media posts, etc.) as a way to convey what we see, hear, and experience to the outside world. The output is then received by other people, and the media they receive influences their own senses.

Another theme of the book is the examination of "tribal" and "industrial" cultures. McLuhan argues that the invention of the media during the Industrial age reconfigured tribal structures. An example he uses is the organization and structure of baseball, hockey, and football games. Each player, member of the coaching staff, general manager, and owner of the team has a specific role in the hierarchy and structure. This collective will then go to into a mock war with other teams. This symbolic warfare mirrors pre-Industrial tribal structures, culture, and political activities.

Yet another theme of the book is the difference between a specialist in a field and a generalist. The author argues that specialists were responsible for creating the industries that sparked the Industrial Revolution. These specialists would later create modern media. While the ideas of the author are scattered, the book, which was published in the 1960s, would anticipate the Internet and the technology revolution of the twenty-first century.