Dickinson opens mid-argument, with an exclamation that sounds like an interruption: "Better -- than Music!" The speaker has heard something and is trying, a little breathlessly, to say what it was, yet she never names the source. All we learn is that she "was used -- to the Birds -- before," and that this sound was a "Translation / Of all tunes I knew -- and more." The word "translation" does a lot of work. It suggests the familiar tunes were already a lesser version of something, and that she has now met whatever they were translated from. The poem is about an experience that outruns art, and about the speaker's stubborn wish to keep hold of it anyway.
The second stanza makes that loss concrete. The sound "Twasn't contained -- like other stanza," so it had no neat borders, and "No one could play it -- the second time." The grammar of the next two lines is knotty. The composer is "perfect Mozart," and the music perishes with him, a "Keyless Rhyme." Whether the composer simply resembles Mozart or really is him, the point is that this is perfection no one can repeat. "Keyless" works two ways: the tune has no musical key, and no key can unlock it. Its beauty cannot be analysed, copied or replayed.
Then the poem swerves into Eden. Children are told that the brooks there "Bubbled a better -- Melody," and they "Quaintly infer" something about Eve's "great surrender," which the poem pictures as "Urging the feet -- that would -- not -- fly." The reasoning is hard to pin down, but the dashes around "not" make that refusal heavy, and the feet sound earthbound. Grown-ups, we're told, are "wiser -- mostly," and the wry "mostly" undercuts that wisdom at once. For adults Eden is "a legend -- dimly told" and Eve's anguish a "Grandame's story." Then the speaker catches herself: "But -- I was telling a tune -- I heard." The sudden self-correction makes the poem feel spoken. It also hints that she will not let her experience be filed away as a quaint tale like Eden.
The fifth stanza makes the boldest claim. The sound is "Not such a strain -- the Church -- baptizes," and no stanza that "splits the silence" when "Redemption strikes her Bells" matches it. Putting it above both church music and the bells of salvation is daring, and the verb "splits" is almost violent. Yet the poem does not turn against faith. The last stanza ends "around the Throne," and the speaker imagines a heaven where her private music will finally fit.
Until then she hums. "Let me not spill -- its smallest cadence" treats sound as liquid, which echoes the bubbling brooks of Eden, and "Humming" repeats twice as if she were doing it as she speaks. Her humming is a "faint Rehearsal," a humble word for something she hopes will "Drop into tune" when the real performance comes.
The form fits this modest, persistent effort. The poem is six quatrains in loose, hymn-like lines of about four beats, with dashes breaking nearly every line into hesitant pieces. The rhymes mostly land on the second and fourth lines. Some are full ("before" and "more," "time" and "Rhyme," "alone" and "Throne"), others only partial ("Aisles" and "Bells"), and in the fourth stanza "told" and "heard" do not rhyme at all. A poem about a tune that cannot be contained keeps its own tidiness slightly loose.
That is also where its significance lies. Dickinson uses poetry, itself a kind of music, to say that the most important sound cannot be written down. She makes that failure the subject, and the poem becomes the "faint Rehearsal" it describes. It is an honest picture of trying to hold on to a moment that language and music can only hint at.