Media are "extensions of man" (or, as we would say today, extensions of people or extensions of humankind) in that they enhance the function of a human body part or faculty. McLuhan writes that it is not just some media or technologies that are extensions of man, but all of them.
Extensions result from the central nervous system's response to stress, but they in turn result in amputation. They alleviate stress, but then cause a new and greater stress as a result, which in tern needs to be alleviated by another extension.
It is only in the electric age that people have become aware of the fact that technologies are extensions of the physical body. McLuhan attributes this to the numbness caused by the extension of the central nervous system itself, though the connection between the two is not clear.