It means that the nature of a medium (the channel through which a message is transmitted) is more important than the meaning or content of the message. McLuhan tells us that a "message" is, "the change of scale or pace or pattern" that a new invention or innovation "introduces into human affairs".
 McLuhan argues that a "message" is, "the change of scale or pace or pattern" that a new invention or innovation "introduces into human affairs".
D
It means that the nature of a medium (the channel through which a message is transmitted) is more important than the meaning or content of the message.
R
McLuhan tells us
McLuhan argues