The most important thing about the title of this poem, "A Poem of Changgan," is "Changgan." What is it? It's an ancient capital city in north central China, so this title clues us into the fact that we're in China.
You can check out "Setting" for more on the geography at work here. The word "Changgan" gives us more than the location, though, it also clues us into the time period. See, in modern day China, the city is referred to as "Xi-an," not as Changgan. Changgan is the old name. So the name also tells us that this poem is set way back in the day, as in, ancient times. In fact, this poem is so ancient, we don't know exactly when it was written, but the poet who wrote it, Li Po, lived between 701 and 762 AD. Yeah, that's over a thousand years ago.
Essentially, then, our title is the poetic equivalent of "A long time ago, in a country far, far away…"