The speaker of the poem has reached a fork in the road in the middle of the woods. He considers the truth that he, "one traveler," cannot take both roads in front of him. The speaker examines one of the roads before him, following it with his eyes as far as he can see, until the road turns slightly into some "undergrowth"—perhaps some tall grasses or small shrubs growing on the forest floor.
Literally, then, to say that the road "bent in the undergrowth" means that, at a certain point, the speaker can no longer see which direction the road goes. The undergrowth obscures the road's direction from the speaker's sight. Moreover, and perhaps more importantly, he cannot see where it leads or where it ends. Symbolically, if we interpret the fork in the road as a choice the speaker must make in his life, then he cannot see what the outcome of his decision will be. Just as he cannot see where the road goes after it bends, he cannot (with certainty) predict where his choice will take him. If we knew the outcomes of our decisions before we made them, that would sure make life a lot easier, would it not? Alas, that is not how it works.