First, we want to point out that "defence" is the British spelling of "defense," but since William Morris was a Brit, you should be careful to spell the word the way he did when you're referring to the title. The same goes for all the character names in the poem – you might be more familiar with alternate spellings of "Guenevere" (Guinevere), "Gauwaine" (Gawain), or "Launcelot" (Lancelot), but you should stick with the spellings Morris used when you're discussing the characters in the context of this poem.
"The Defence of Guenevere" is a deceptively straightforward title. The name "Guenevere" immediately evokes legends of King Arthur and the Knights of the Round Table, chivalry, fighting evil, Monty Python, all that good stuff. But the name also evokes the legend of Guenevere's scandalous love affair with Launcelot, one of King Arthur's knights. So the "Defence of Guenevere" could mean that the poet is leaping to Guenevere's defense, offering excuses, or it could describe physical combat to defend her. In fact, though, the "Defence of Guenevere" described in the title is Guenevere's own, verbal defense of herself. She is on trial, and as an accused adulteress, she is allowed to speak in her own defense. And boy, does she.