In Hatchet, Brian's survival depends on his ability to figure out how to take care of himself: how to find food and shelter, how to avoid being attacked by a dangerous animal, how to hold out until he is rescued. You know, the usual.
The knowledge that he needs would have been common just a few hundred years ago, but he's forced to patch together bits and scraps of information gleaned from books, TV shows, classes in school, anything he can think of. Knowledge, in the "civilized" world of Brian's past, usually comes from second-hand sources; it's something someone tells you. In the world of the woods, on the other hand, knowledge is first-hand and largely experiential; that is, it comes from direct experience. The only way Brian can learn how to build a fire, or catch a fish, is by actually doing it. And, most often, by doing it again and again… and again. Practice makes perfect, right?
In Hatchet, real knowledge can only be gained through experience.
Hatchet suggests that modern man has lost an understanding of who he really is, an understanding that can only be gained through immersion in the natural world.