This poem is a dramatic monologue, spoken by Mary Webster. As Atwood says in a short introduction, Webster was one of many 17th-century victims of the Puritan witch-hunts. A widow from Massachusetts, Webster was accused of witchcraft and hanged by her fellow citizens—but survived her hanging. In this poem, she'll tell her own story in the first person.
"Half-hanged Mary" itself then begins with an ominous moment of personification: "Rumour," the speaker says, "was loose in the air, / hunting for some neck to land on." That predatory "rumour" is about to come for the speaker herself, the word "neck" foreshadowing the hanging to come.
These first lines evoke the claustrophobic, gossipy danger of a small 17th-century town. In Mary's world, it seems, words have a lot of power: the mere "rumour" that Mary's a witch seals her fate.
Take a look at Mary's simile in the second stanza here:
I didn't feel the aimed word hit
and go in like a soft bullet.
The "bullet" of the "aimed word"—witch—hits so "soft[ly]" that Mary doesn't even know that she's doomed; her "smashed flesh" swallows that bullet as easily as "water" closes over a "thrown stone." But it takes only that one "aimed word" to ruin her life.
But here at "7 p.m.," the outset of Mary's ordeal, she has no idea what she's about to face. She's just placidly "milking the cow" in her barn, minding her own business.
These first stanzas of the poem give readers a preview of the way Mary will shape her story. The poem will chart her suffering like a crime procedural, using sections labeled with times of day to follow her, hour by hour, as she's captured and hanged. And she'll use loose, flexible free verse to tell her tale—a choice that reflects her independence and willpower. She won't fit her language into any particular poetic form: she'll bend form to suit her meaning.