This is a chapter that I have struggled to understand in the past. Although I understood sombunall (as RAW likes to put it), this particular sentence seems to describe the main message:
"We know the model of the "external universe" inside our brains, which exist inside our heads."
This right here is a key to the statement, "The map is not the territory." At any given moment, we are only interacting with a model that our nervous system has created of our exterior surroundings. When I see an object, I am not actually sending some kind of energy out of my eyes and determining that qualities of said object. Instead, I am actually receiving the photons given off by that object into my eye, which is then performing complex processes in the brain to convert that signal into what I perceive. Even more interesting, when the eye receives these photons, the "image" that gets reconstructed is upside down, and it is my brain that flips the image so that it is "right-side up."
Similarly, with taste, touch, smell, and sound, all of these signals are being received by the body and reconstructed into a gestalt that hopefully is accurate to what we call reality. Anyone who has spent enough time with these ideas will quickly realize there are so many different ways this can go wrong, and our perceptions can be inaccurate. Hence, RAW's assertion that we are taking a gamble every time we interpret sensory information. This destroys any notion of "objectivity" or "absolutism" that humans like to assert power over.
Naturally, since these processes are exactly that, processes, we are able to train ourselves to do these processes better. While none of our perceptions are "objective," some of our perceptions might hold up more to reality testing. In fact, I'd venture to guess that someone who is closer to reality might be (at the least) intuitively aware of these ideas.
Upon this reading of the chapter, I can appreciate that RAW communicates some of the complexities of Neuroscience and Psychophysics to those who may not have any experience in a Psychology class at a college. Even as Neuroscience has evolved dramatically since RAW's writing of this chapter, it still holds up pretty well.