I agree with the previous poster. I would also add that an important part of Thoreau's philosophy is reflection. He encourages us to withdraw along with him, at least for a short time, and reevaluate what is important in life.
You might be able to use this quotation as an introduction to a very personal essay, one in which you talk about how you find some things in life to be distracting and perhaps difficult to resist (e.g. worrying all the time about doing the right things to be popular or preparing to study a subject that you don't really find interesting but expect to be able to earn good money with) and other things being more important, although they might be more easily overlooked. Thoreau's own solution, of course, is not to completely withdraw into the woods for the rest of his life. He praises the "half-cultivated bean field" and returns to civillization after just a short time in the woods.
The answer to this sort of depends on what point you're trying to make in your essay or what you've been asked to write about. If you are just trying to explain that quote or use that quote to talk about what Thoreau is saying, I'd say something like this:
In Walden, one thing Henry David Thoreau is trying to do is convince people to live simply. He argues that people get too caught up in the things they have and forget about trying to be happy rather than rich. He condemns this kind of materialism, saying "we do not ride on the railroad; it rides upon us."