The speaker of this poem is clearly a nature lover. There he is (and we're just assuming that he's a… he), up in the mountains somewhere all by himself, taking in the view of a valley at sunset. It's postcard-perfect: there's a castle, a lake, a waterfall—he describes it all. But what he sees is not the most interesting thing to our speaker. Nope. He's more interested in what he can hear.
And what he hears is the "thin and clear" (7) sound of a bugle, echoing from… well, from somewhere. We're not sure where, or even who this mysterious bugler even is. What we do know, though, is the impact that the bugle's echoes have on our speaker's thinking.
The sound of the bugle seems to spark our speaker's imagination: he imagines that the horns are coming from "Elfland," and that there's something magical about them. It's as though these echoes transport him to a different world than the one he's standing in. But that transportation doesn't stop there. From Elfland, he's off again, thinking—as the echoes of the horn die away— about other kinds of "echoes," the legacy, or "echo," that people leave behind them when they die.
Cheery, right? Still, this shift in attention tells us that we're dealing with an introspective person, one whose mind is agile enough to jump from natural vistas to fantastic realms to thoughts of death. Also, he's attuned enough to his surroundings to be sent off in that direction by a distant sound. Finally, we get the sense that, despite the gloomy Gus-side of his reflecting, the speaker is getting a lot out of this exercise. That would explain why he seems to command that bugle to "blow, and set the wild echoes flying" (5, 17).