The speaker here describes a mysterious “They” as trying to limit her to writing in prose (“They shut me up in Prose –“). She compares this to when, as a child (“As when a little Girl”), she was put in the closet in an effort to keep her still and quiet (“They put me in the Closet -- / Because they liked me ‘still’ – “).
She then scoffs at this idea—“Still!”—because if they had been able to see inside her mind while she was in there (“Could themself have peeped -- / And seen my Brain – go round –“) it would have been clear to them that they had not successfully quieted her at all. In fact, their attempt to imprison her was as foolish as imprisoning a bird for treason in the pound.
This bird has only but to will itself free (“Himself has but to will”), and he becomes free (“Abolish his Captivity”), as “easy as a Star.” It is so easy that he can laugh at their foolishness. The speaker, just as easily, can will herself out of their captivity (“No more have I –“), because they cannot control her mind.
“They shut me up in Prose –“ is a famously rebellious poem. Although the speaker’s gender may be inconsequential, and this only a poem of the rebellion of the artist, it seems more likely that it is a poem about the predicament of the female artist—or maybe even just the female. That the speaker’s gender is indeed important is clear in that she specifies it, closely associated with her captivity, in the second line—she was a “little Girl.”
From that moment on, it is hard to separate the attempted constraints on her from her gender. They liked her “still” because little girls were meant to be seen and not heard, to be quiet and docile, well-mannered and pretty. The implication here, if the speaker had to be contained in a closet, is that she was none of these things—she was, instead, rebellious. The use of the word “themself” emphasizes the strength required for such a rebellion, for the use of this instead of the more grammatically correct themselves makes the “they” into a single entity, and thus the reader cannot imagine anyone siding with the speaker, except for herself.
Her rebelliousness cannot be quashed with captivity, however. No matter how they try to contain her or imprison her, she, like a bird, “has but to will” and she can “Abolish” her captivity, for they cannot contain or quiet her mind, they cannot control what she thinks. In fact, the captivity they impose on her body actually frees her mind, revs it up—in the closet, her brain goes “round” at an even faster pace, we imagine. The metaphor of the bird does not just emphasize her ability to withstand their imprisonment, but also her innocence, for surely a bird could not be guilty of treason, just as a little girl should not be considered guilty for her individuality.
The first line of this poem, however, makes this about more than gender. It is also about art. A mysterious “They,” society, most likely, does not shut the adult speaker up in the closet, but “in Prose.” This is not the only poem in which Dickinson compares prose unfavorably to poetry, specifically because poetry is more open, more free. Perhaps, here, they want Dickinson to write only letters, correspondence, not to try her hand at the male-dominated art of poetry, which she uses to such powerful effect.
This poem’s tone is scornful—the bird laughs at its captors, Dickinson scoffs at them for thinking they could keep her “Still!”—and the poem itself is a part of that scoff. For the very existence of this poem shows that “They” have failed, and is thus a kind of ironic defiance. They have not shut her up in prose, they have in fact only inflamed and inspired her, by trying to keep her captive, to write this very poem.