Dickinson's poem is, of course, about poets and their art. Poets are able to distill a heady fragrance ("Attar") from "the familiar species / That perished by the Door." This can mean that poets turn everyday things into objects filled with meaning, but it is characteristic of Dickinson to use an oblique image of death to make this point: it's not clear what "familiar species" has "perished," but the idea is that out of the triviality of death, the poet is able to make something sublime. This split between the ordinariness of life and the heightened experience of poetry is returned to in the third and fourth stanzas. In the third stanza, the poet ("the Discloser") impoverishes ("entitles") everyone else ("us") through the power of their "picture" or imagery. This suggests a second sort of split—an internal one within Dickinson herself (her use of "us" suggests that the poet Dickinson is at odds with the person Dickinson, who is like the reader). In the final stanza, the work of the poet is at once a kind of "robbery," but one the "us" from the third stanza are "unconscious" of.
Looking at the final stanza in detail:
Of portion—so unconscious—
This line refers to the "portion" of meaning the poet has discovered, of which most people are unconscious (or the meaning itself is unconscious of its value, perhaps).
The Robbing—could not harm—
"Robbing" I read as the poetic act, the extraction of meaning from everyday things.
Himself—to Him—a Fortune—
The "himself" refers to Dickinson the person, one of "us" who is not "harmed" by the "robbery," while the "him" is the Poet (Dickinson, despite the masculine pronoun) who has stolen a "Fortune" in meaning.
This "Fortune" is "exterior" to time—that is, beyond death—because it will endure as art.