To be sure, many of those nature poems are also about… death, religious doubt, and unrequited love, but this one seems to be pretty much "about" the way that nature can stop you in your tracks and force you to pay attention. Dickinson spent a lot of time alone, and for much of her adulthood her only companion was her dog, Carlo. Sweet, but sad. In any case, perhaps for that reason many of her poems portray things that one could see while on a walk.
The poem was originally included in a letter to Dickinson's friend, mentor, and possible love-interest Thomas Higginson in 1880. She prefaced the poem with the words, "Dear friend, to the oriole you suggested I add a hummingbird and hope they are not untrue." This sentence provides the scholarly proof that the poem is about a hummingbird. Lucky us! Were it not for this letter, we may not have ever known with certainty what the poem is "about." The poem's origin is important, because while we may read the poem as a separate entity, it was originally attached to something that gave it meaning and context.
"A Route of Evanescence" is an important Dickinson poem to read because it takes a simple, single event—a hummingbird flying near some flowers—and describes that event in extremely condensed and difficult language. In fact, it may be one of the hardest 8-line poems ever written (lucky you!). At least, it's definitely the hardest poem about a hummingbird ever written. That makes it classic Dickinson.
Dickinson's speakers often zoom in on details that ordinary people might not see because they're too busy, you know, working, raising kids, or doing other "normal" social activities. "A Route of Evanescence" is about the way that nature can encourage us to slow down, though, stop all that working and thinking, and just, you know, look.
In today's hyper-technological world, it can be hard to connect with nature, especially if you live in a big city. Dickinson wants to challenge us to find a way, though. We may not be stopping to smell the roses—or even snap photos of them with our smart phones—but at the very least we can notice how cool it is that hummingbirds can fly backwards and flap their wings up to 90 times per second!