Lines 1-6 introduce the poem's speaker: Eurydice, whose name is the poem's title. Eurydice is a well-known character from ancient Greek mythology, a young woman who dies tragically and winds up in the afterlife, or "Underworld" (line 2).
In the original myth, Orpheus, her loving husband, descends to the underworld in a doomed attempt to bring Eurydice back to life. But Carol Ann Duffy's version of this myth is very different from the original. Rather than a romantic tragedy, it's a feminist satire. It's also a dramatic monologue: a poem whose speaker is a character recognizably different than the poet.
In these opening lines, Eurydice narrates her experience as a ghost to an unidentified group of "Girls." Eurydice could be addressing a group of other dead women, or this could be an apostrophe to female readers/listeners in general. Either way, that first word marks the poem's intended audience—or primary audience, at least—as "Girls" and women. This will be a feminist retelling of a myth that traditionally places a man (Orpheus) in the hero's role. The casual tone sets up an intimacy between speaker and audience, as if Eurydice were confiding the real story of her life and death to a gathering of girlfriends.
She describes her post-death "self" as a "shade" or "shadow," following traditional depictions of the afterlife in Greek myth. ("Shade" is a common English translation of the ancient Greek word for spirits in the underworld; it means roughly the same as "ghost.") She also describes the afterlife as a world outside of time: a "nowhen." (Compare the word "nowhere" for place.)
Most importantly, she stresses how silent the afterlife is; she calls it "a place where language stopped." Metaphorically, it's "a black full stop"—a period at the end of life's sentence—or a "black hole" that swallows up "words" entirely. Eerie as these comparisons might sound to the reader, Eurydice soon makes clear that she loves the silence! She especially treasures it in comparison to the rambling poetry of Orpheus.
Alliteration ("dead and down," "a shade, / a shadow," "where the words") and repetition ("a black full stop, a black hole") add spice and emphasis to Eurydice's language. As she tells her story to a group of listeners, real or imagined, these devices help hook the audience's attention.