We know that Dickinson didn't title her poems, so if you were to ask her a question like, "What's Up With the Title?", she might be all, "Um, what title?" Then things might get awkward. You see, the thing is that Dickinson didn't include titles, in part because she didn't intend them to be read by a public audience (though many of them were mailed to friends). In collections, her poems are primarily organized by the order in which the editor thinks they were written. For example, this poem is listed as #1463 in the popular 1955 Thomas Johnson edition of her poems. For purposes of discussing the poem in this module, we refer to it by its first line, "A Route of Evanescence."
So, it might be worth asking instead, "What's Up With the First Line?" Now we've got something because, even though Dickinson didn't include a title, her choice of first lines is important in establishing her poetic intentions. That's especially true since most of her poetry is very short. So, she needs to get to the point quick-fast in a hurry.
In this case, how does "A Route of Evanescence" as a line work to set us readers up for the rest of the poem to come? There are a lot of ways to answer that question, really. One is that, this line describes a path, a route, which would indicate a means by which journeys are taken. It's hard to take a trip, really, without a route—even with one of those fancy GPS devices. (Just go ahead and try it, why don't you?) In this case, then, we have a means to a journey, but that means (the route) is not substantial. It's a path of evanescence (and we don't mean that cheesy metal band), which means that it's insubstantial, temporary, and fading.
Another way to put it is that the route is more of suggestion, a kind of impression—as opposed to an eight-lane superhighway. Now, how would that be appropriate for a reader just dipping her toe into the waters of this poem? A poem whose sole purpose is to record the path that a fleeting impression (of a hummingbird in flight) leaves in the speaker's mind? See where we're headed with this? The first line really announces what the poem's all about. We're about to be taken on a journey that records a vague, though powerful, impression in abstract terms. In other words, we're off on a route of evanescence!