The poem opens on a mysterious "route," a path or a road from one place to another. We don't know where we've come from or where we are going. We only learn a few things about this path.
First, it's "obscure." That means dark and dim, but also hard to find, remote, or distant. "Obscure" is a great Poe word, full of the spooky mystery that's everywhere in this poem.
He also tells us that the road is "lonely." That sense of being alone and abandoned is another big theme in this poem (and in a lot of Poe's work).
In just a few words, without telling us much of anything specific, Poe has set the mood for the entire poem.