Sylvia Plath was a master of metaphor (in fact, she wrote a well-known poem called "Metaphors"), and "Edge" displays her flair for intense figurative language. Lines 4-5, for example, combine metaphor and allusion in a dense, complex statement:
The illusion of a Greek necessity
Flows in the scrolls of her toga,
The "toga" the woman is wearing appears to be literal (though it could be a fancy description of a more modern type of robe). A toga is a loose, flowing garment that was commonly worn in ancient Greece and Rome. Here, its loops and folds are compared to "scrolls" (rolled or spiral shapes), a word that invokes the old-fashioned parchment scrolls that were once used as writing paper. (If this woman is supposed to be a version of Plath, this word might point toward her vocation as a writer.) The woman's "Flow[ing]" toga makes her look like a tragic heroine from ancient "Greek" drama, in which fate, or "necessity," often dictated characters' deaths.
Lines 9-10 compare the children at the woman's breasts to "serpents," because of the way they're curled up, or "coiled" (and also as an allusion to Shakespeare's Antony and Cleopatra, whose heroine holds a snake to her breast to kill herself). Her breasts are compared to "little / Pitchers of milk, now empty," meaning that she was nursing her children, but her milk has dried up now that she and they have died.
Notice that these metaphors seem to clash slightly, because snakes aren't mammals and don't nurse! But Plath often squashed unlikely metaphors and images together for the sake of dreamlike intensity. Notice how the image immediately changes again in the simile spanning lines 12-16: suddenly, the "serpent[s]" are petals of a rose, which is closing up in the nighttime.
Two more figurative phrases appear in the final lines, as part of an anthropomorphic description of the moon. The speaker imagines this moon as a woman looking out from beneath a "hood of bone": a metaphorical description of a bone-white lunar crescent. "Her blacks crackle and drag" seems to compare the night sky to a crackling cape, or similar garment, that the moon is dragging in her wake.