In Frost's "The Road Not Taken," what do the woods symbolize?

In this poem, the narrator comes to a fork in the road, and he must decide which road to take. Both forks run in different directions through a "yellow wood." It's significant that the narrator stands in the woods, rather than on an open plain or a mountaintop, because the woods make it difficult to see. He cannot scan the countryside for miles and miles in every direction, as he could on the open prairies. Not only do the trees trunks themselves obscure his view, but the trees also must cast shadows that darken the woods. 

The woods thus symbolize the difficulty of seeing into the future. The narrator must make a choice based on limited information, since much of what he might like to see is obscured. The narrator will have to rely on faith and intuition as he makes his choice, as we often must do in life.