@dan hassan
I went through a McLuhan phase in college & read Gutenberg Galaxy, Understanding Media, Mechanical Bride, & a couple others. Explanations in this thread have been pretty good, but there's a big aspect that I haven't seen mentioned (though I skimmed some comments so I might have missed it).

Specifically: different media have different biases about what kinds of messages can be more easily transmitted & recieved through them, and at scale, these biases are inherited by people who get most of their information through said media.

A trivial example: radio is not very good at transmitting pictures -- the best one can do is give a physical description -- so anything visual that's also hard to describe in words simply won't be understood by avid radio listeners.

A more nuanced example (and one that McLuhan used a lot without really explaining properly -- one that's confusing today) is the difference between film & the kind of broadcast TV that people had access to in the 60s. To watch a film, you go into a darkened theatre, and there, much larger than life, is an extremely high-fidelity image (until quite recently it wasn't possible to get the kind of fidelity in digital images that you can with film), and you see (generally speaking) a complete story of about two hours, uninterrupted. Everything on film is glamorous in a way, simply because it's very big & bright & you can see more detail than you normally would if you were seeing the thing in reality because an expert is guiding your focus and attention to only important things. Meanwhile, in the 60s, a TV screen would have been quite small, it probably wouldn't have been in color but if it was you'd get color bleed because of the way color is encoded in analog TV signals, and radio interference would make it so the image was often so noisy that it was impossible to say with certainty what was supposed to be on the screen; TV broadcasts tend to tell much shorter stories, interrupted by ads, where the audience isn't necessarily expected to see the whole episode let alone previous ones. Therefore, the two media are good for different things: because of how much more information you get with film, you can have the idea of 'auteur' films -- a filmmaker can only be seen to be in total control of a film because a film has almost total control over its audience -- and at the same time, the messiness of television means that TV viewers need to use their imaginations & really give TV the benefit of the doubt, putting more of themselves into the TV watching experience & seeing reflected back stuff they themselves imagined.

This sort of extends to power relations. If a certain medium is dominant, then the people who are best able to consume that medium are the best-informed people & the people who are best able to create it are the most culturally powerful people. The shift from orality to literacy didn't just mean that stuff that had previously been in flux becam solidified as a canon, but also that power shifted from people who could memorize large amounts of poetry and recite it entertainingly to people who were able to sit for long periods of time, read without moving their lips, write legibly, and afford manuscripts. Print accelerated some of these things but not others: books got cheaper and writing legibly mattered less, but the availability of books meant literacy became more common & more readers meant more writers which in turn meant a greater variety of written material, so it mattered a lot more whether or not you could read quickly and for a long period of time. Print brought with it changing norms around scholarship: since references could now be checked, your authority as an academic depended on checking and citing references, and this led to the development of new media within printed writing with their own biases in both power and information flow: alphabetical indexes, library catalogues, dictionaries, collections of epigrams, eventually encyclopedias and card catalogues and numeric cataloguing systems...