The poem opens with an epigraph:
Item I gyve unto my wief my second best bed…
This is a quote taken from William Shakespeare's will, in which the playwright leaves his "second best bed" to his wife. While this sounds like an insult, it probably wasn't: in Shakespeare's day, the "best bed" was usually reserved for visitors and later bequeathed to one's children/heirs. The "second best bed," here, is actually the bed that Shakespeare and his wife, Anne Hathaway, would have shared!
Using the bed as a kind of launchpad for her fond memories, the widowed Hathaway wastes no time in portraying her marriage as a loving, passionate one:
The bed we loved in was a spinning world
This metaphor depicts the couple's marital bed as an exciting, intensely intimate place—an entire world unto itself, one that revolves on its own axis. (The "spinning world" also probably alludes to the Globe Theatre, where many of Shakespeare's plays were first performed.) Through their lovemaking, the couple was figuratively transported to "forests" and "castles"; to worlds lit up by "torchlight"; to the tops of "cliffs" and even into the ocean. Love and sex, here, are like writing: able to break open the imagination and conjure up entire worlds.
Not coincidentally, all of these locations appear in Shakespeare's plays: readers might think of Elsinore, the royal castle of Hamlet; the idyllic Forest of Arden from As You Like It; the foreboding cliffs of King Lear. Of course, lots of other plays fit the bill here too! Indeed, the asyndeton in the line—that is, the lack of an and—suggests these are just a few imagined places among many. The important point is that the poem establishes a link between passionate love and creative work.
Hathaway also refers to "seas / where [Shakespeare] would dive for pearls." This is, in all likelihood, a sensuous euphemism for oral sex. It's also probably an allusion to The Tempest—more specifically, to a famous song sung by the spirit Ariel:
Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes;
Nothing of him that doth fade,
But doth suffer a sea-change
Into something rich and strange.
This song is about change and transformation, again suggesting that lovemaking, too, can be a transformative act.
Finally, these opening lines establish the poem's meter: Duffy uses a lose iambic pentameter here, meaning there are five iambs (feet with a da-DUM rhythm) per line. The meter isn't perfect—even line 1 has a variation on the third foot ("in was"):
The bed | we loved | in was | a spin- | ning world
Still, the iambic rhythm is pretty clear. Shakespeare famously used iambic pentameter in his own sonnets, and its appearance here thus links the poem to Shakespeare through both its images and its form.